gecko-dev/servo/components/style/stylist.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

//! Selector matching.

use {Atom, LocalName, Namespace, WeakAtom};
use applicable_declarations::{ApplicableDeclarationBlock, ApplicableDeclarationList};
use context::{CascadeInputs, QuirksMode};
use dom::{TElement, TShadowRoot};
use element_state::{DocumentState, ElementState};
use font_metrics::FontMetricsProvider;
#[cfg(feature = "gecko")]
use gecko_bindings::structs::{ServoStyleSetSizes, StyleRuleInclusion};
use hashglobe::FailedAllocationError;
use invalidation::element::invalidation_map::InvalidationMap;
use invalidation::media_queries::{EffectiveMediaQueryResults, ToMediaListKey};
#[cfg(feature = "gecko")]
use malloc_size_of::{MallocShallowSizeOf, MallocSizeOf, MallocSizeOfOps};
#[cfg(feature = "gecko")]
use malloc_size_of::MallocUnconditionalShallowSizeOf;
use media_queries::Device;
use properties::{self, CascadeMode, ComputedValues};
use properties::{AnimationRules, PropertyDeclarationBlock};
use rule_cache::{RuleCache, RuleCacheConditions};
use rule_tree::{CascadeLevel, RuleTree, ShadowCascadeOrder, StrongRuleNode, StyleSource};
use selector_map::{PrecomputedHashMap, SelectorMap, SelectorMapEntry};
use selector_parser::{PerPseudoElementMap, PseudoElement, SelectorImpl, SnapshotMap};
use selectors::NthIndexCache;
use selectors::attr::{CaseSensitivity, NamespaceConstraint};
use selectors::bloom::{BloomFilter, NonCountingBloomFilter};
use selectors::matching::{matches_selector, ElementSelectorFlags, MatchingContext, MatchingMode};
use selectors::matching::VisitedHandlingMode;
use selectors::parser::{AncestorHashes, Combinator, Component, Selector};
use selectors::parser::{SelectorIter, Visit};
use selectors::visitor::SelectorVisitor;
use servo_arc::{Arc, ArcBorrow};
use shared_lock::{Locked, SharedRwLockReadGuard, StylesheetGuards};
use smallbitvec::SmallBitVec;
use smallvec::SmallVec;
use std::ops;
use std::sync::Mutex;
use style_traits::viewport::ViewportConstraints;
use stylesheet_set::{DataValidity, DocumentStylesheetSet, SheetRebuildKind};
use stylesheet_set::{DocumentStylesheetFlusher, SheetCollectionFlusher};
#[cfg(feature = "gecko")]
use stylesheets::{CounterStyleRule, FontFaceRule, FontFeatureValuesRule, PageRule};
use stylesheets::{CssRule, Origin, OriginSet, PerOrigin, PerOriginIter};
use stylesheets::StyleRule;
use stylesheets::StylesheetInDocument;
use stylesheets::keyframes_rule::KeyframesAnimation;
use stylesheets::viewport_rule::{self, MaybeNew, ViewportRule};
use thread_state::{self, ThreadState};

/// The type of the stylesheets that the stylist contains.
#[cfg(feature = "servo")]
pub type StylistSheet = ::stylesheets::DocumentStyleSheet;

/// The type of the stylesheets that the stylist contains.
#[cfg(feature = "gecko")]
pub type StylistSheet = ::gecko::data::GeckoStyleSheet;

/// A cache of computed user-agent data, to be shared across documents.
lazy_static! {
    static ref UA_CASCADE_DATA_CACHE: Mutex<UserAgentCascadeDataCache> =
        Mutex::new(UserAgentCascadeDataCache::new());
}

struct UserAgentCascadeDataCache {
    entries: Vec<Arc<UserAgentCascadeData>>,
}

impl UserAgentCascadeDataCache {
    fn new() -> Self {
        Self { entries: vec![] }
    }

    fn len(&self) -> usize {
        self.entries.len()
    }

    // FIXME(emilio): This may need to be keyed on quirks-mode too, though there
    // aren't class / id selectors on those sheets, usually, so it's probably
    // ok...
    fn lookup<'a, I, S>(
        &'a mut self,
        sheets: I,
        device: &Device,
        quirks_mode: QuirksMode,
        guard: &SharedRwLockReadGuard,
    ) -> Result<Arc<UserAgentCascadeData>, FailedAllocationError>
    where
        I: Iterator<Item = &'a S> + Clone,
        S: StylesheetInDocument + ToMediaListKey + PartialEq + 'static,
    {
        let mut key = EffectiveMediaQueryResults::new();
        debug!("UserAgentCascadeDataCache::lookup({:?})", device);
        for sheet in sheets.clone() {
            CascadeData::collect_applicable_media_query_results_into(device, sheet, guard, &mut key)
        }

        for entry in &self.entries {
            if entry.cascade_data.effective_media_query_results == key {
                return Ok(entry.clone());
            }
        }

        let mut new_data = UserAgentCascadeData {
            cascade_data: CascadeData::new(),
            precomputed_pseudo_element_decls: PrecomputedPseudoElementDeclarations::default(),
        };

        debug!("> Picking the slow path");

        for sheet in sheets {
            new_data.cascade_data.add_stylesheet(
                device,
                quirks_mode,
                sheet,
                guard,
                SheetRebuildKind::Full,
                Some(&mut new_data.precomputed_pseudo_element_decls),
            )?;
        }

        let new_data = Arc::new(new_data);
        self.entries.push(new_data.clone());
        Ok(new_data)
    }

    fn expire_unused(&mut self) {
        self.entries.retain(|e| !e.is_unique())
    }

    fn clear(&mut self) {
        self.entries.clear();
    }

    #[cfg(feature = "gecko")]
    fn add_size_of(&self, ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
        sizes.mOther += self.entries.shallow_size_of(ops);
        for arc in self.entries.iter() {
            // These are primary Arc references that can be measured
            // unconditionally.
            sizes.mOther += arc.unconditional_shallow_size_of(ops);
            arc.add_size_of(ops, sizes);
        }
    }
}

/// Measure heap usage of UA_CASCADE_DATA_CACHE.
#[cfg(feature = "gecko")]
pub fn add_size_of_ua_cache(ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
    UA_CASCADE_DATA_CACHE
        .lock()
        .unwrap()
        .add_size_of(ops, sizes);
}

type PrecomputedPseudoElementDeclarations = PerPseudoElementMap<Vec<ApplicableDeclarationBlock>>;

#[derive(Default)]
struct UserAgentCascadeData {
    cascade_data: CascadeData,

    /// Applicable declarations for a given non-eagerly cascaded pseudo-element.
    ///
    /// These are eagerly computed once, and then used to resolve the new
    /// computed values on the fly on layout.
    ///
    /// These are only filled from UA stylesheets.
    precomputed_pseudo_element_decls: PrecomputedPseudoElementDeclarations,
}

impl UserAgentCascadeData {
    #[cfg(feature = "gecko")]
    fn add_size_of(&self, ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
        self.cascade_data.add_size_of(ops, sizes);
        sizes.mPrecomputedPseudos += self.precomputed_pseudo_element_decls.size_of(ops);
    }
}

/// All the computed information for a stylesheet.
#[derive(Default)]
#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
struct DocumentCascadeData {
    #[cfg_attr(feature = "servo",
               ignore_malloc_size_of = "Arc, owned by UserAgentCascadeDataCache")]
    user_agent: Arc<UserAgentCascadeData>,
    user: CascadeData,
    author: CascadeData,
    per_origin: PerOrigin<()>,
}

/// An iterator over the cascade data of a given document.
pub struct DocumentCascadeDataIter<'a> {
    iter: PerOriginIter<'a, ()>,
    cascade_data: &'a DocumentCascadeData,
}

impl<'a> Iterator for DocumentCascadeDataIter<'a> {
    type Item = (&'a CascadeData, Origin);

    fn next(&mut self) -> Option<Self::Item> {
        let (_, origin) = self.iter.next()?;
        Some((self.cascade_data.borrow_for_origin(origin), origin))
    }
}

impl DocumentCascadeData {
    fn borrow_for_origin(&self, origin: Origin) -> &CascadeData {
        match origin {
            Origin::UserAgent => &self.user_agent.cascade_data,
            Origin::Author => &self.author,
            Origin::User => &self.user,
        }
    }

    fn iter_origins(&self) -> DocumentCascadeDataIter {
        DocumentCascadeDataIter {
            iter: self.per_origin.iter_origins(),
            cascade_data: self,
        }
    }

    fn iter_origins_rev(&self) -> DocumentCascadeDataIter {
        DocumentCascadeDataIter {
            iter: self.per_origin.iter_origins_rev(),
            cascade_data: self,
        }
    }

    /// Rebuild the cascade data for the given document stylesheets, and
    /// optionally with a set of user agent stylesheets.  Returns Err(..)
    /// to signify OOM.
    fn rebuild<'a, S>(
        &mut self,
        device: &Device,
        quirks_mode: QuirksMode,
        mut flusher: DocumentStylesheetFlusher<'a, S>,
        guards: &StylesheetGuards,
    ) -> Result<(), FailedAllocationError>
    where
        S: StylesheetInDocument + ToMediaListKey + PartialEq + 'static,
    {
        // First do UA sheets.
        {
            if flusher.flush_origin(Origin::UserAgent).dirty() {
                let mut ua_cache = UA_CASCADE_DATA_CACHE.lock().unwrap();
                let origin_sheets = flusher.origin_sheets(Origin::UserAgent);
                let ua_cascade_data =
                    ua_cache.lookup(origin_sheets, device, quirks_mode, guards.ua_or_user)?;
                ua_cache.expire_unused();
                debug!("User agent data cache size {:?}", ua_cache.len());
                self.user_agent = ua_cascade_data;
            }
        }

        // Now do the user sheets.
        self.user.rebuild(
            device,
            quirks_mode,
            flusher.flush_origin(Origin::User),
            guards.ua_or_user,
        )?;

        // And now the author sheets.
        self.author.rebuild(
            device,
            quirks_mode,
            flusher.flush_origin(Origin::Author),
            guards.author,
        )?;

        Ok(())
    }

    /// Measures heap usage.
    #[cfg(feature = "gecko")]
    pub fn add_size_of(&self, ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
        self.user.add_size_of(ops, sizes);
        self.author.add_size_of(ops, sizes);
    }
}

/// Whether author styles are enabled.
///
/// This is used to support Gecko.
#[allow(missing_docs)]
#[derive(Clone, Copy, Debug, Eq, MallocSizeOf, PartialEq)]
pub enum AuthorStylesEnabled {
    Yes,
    No,
}

/// A wrapper over a DocumentStylesheetSet that can be `Sync`, since it's only
/// used and exposed via mutable methods in the `Stylist`.
#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
struct StylistStylesheetSet(DocumentStylesheetSet<StylistSheet>);
// Read above to see why this is fine.
unsafe impl Sync for StylistStylesheetSet {}

impl StylistStylesheetSet {
    fn new() -> Self {
        StylistStylesheetSet(DocumentStylesheetSet::new())
    }
}

impl ops::Deref for StylistStylesheetSet {
    type Target = DocumentStylesheetSet<StylistSheet>;

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl ops::DerefMut for StylistStylesheetSet {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}

/// This structure holds all the selectors and device characteristics
/// for a given document. The selectors are converted into `Rule`s
/// and sorted into `SelectorMap`s keyed off stylesheet origin and
/// pseudo-element (see `CascadeData`).
///
/// This structure is effectively created once per pipeline, in the
/// LayoutThread corresponding to that pipeline.
#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
pub struct Stylist {
    /// Device that the stylist is currently evaluating against.
    ///
    /// This field deserves a bigger comment due to the different use that Gecko
    /// and Servo give to it (that we should eventually unify).
    ///
    /// With Gecko, the device is never changed. Gecko manually tracks whether
    /// the device data should be reconstructed, and "resets" the state of the
    /// device.
    ///
    /// On Servo, on the other hand, the device is a really cheap representation
    /// that is recreated each time some constraint changes and calling
    /// `set_device`.
    device: Device,

    /// Viewport constraints based on the current device.
    viewport_constraints: Option<ViewportConstraints>,

    /// The list of stylesheets.
    stylesheets: StylistStylesheetSet,

    /// If true, the quirks-mode stylesheet is applied.
    #[cfg_attr(feature = "servo", ignore_malloc_size_of = "defined in selectors")]
    quirks_mode: QuirksMode,

    /// Selector maps for all of the style sheets in the stylist, after
    /// evalutaing media rules against the current device, split out per
    /// cascade level.
    cascade_data: DocumentCascadeData,

    /// Whether author styles are enabled.
    author_styles_enabled: AuthorStylesEnabled,

    /// The rule tree, that stores the results of selector matching.
    rule_tree: RuleTree,

    /// The total number of times the stylist has been rebuilt.
    num_rebuilds: usize,
}

/// What cascade levels to include when styling elements.
#[derive(Clone, Copy, PartialEq)]
pub enum RuleInclusion {
    /// Include rules for style sheets at all cascade levels.  This is the
    /// normal rule inclusion mode.
    All,
    /// Only include rules from UA and user level sheets.  Used to implement
    /// `getDefaultComputedStyle`.
    DefaultOnly,
}

#[cfg(feature = "gecko")]
impl From<StyleRuleInclusion> for RuleInclusion {
    fn from(value: StyleRuleInclusion) -> Self {
        match value {
            StyleRuleInclusion::All => RuleInclusion::All,
            StyleRuleInclusion::DefaultOnly => RuleInclusion::DefaultOnly,
        }
    }
}

impl Stylist {
    /// Construct a new `Stylist`, using given `Device` and `QuirksMode`.
    /// If more members are added here, think about whether they should
    /// be reset in clear().
    #[inline]
    pub fn new(device: Device, quirks_mode: QuirksMode) -> Self {
        Self {
            viewport_constraints: None,
            device,
            quirks_mode,
            stylesheets: StylistStylesheetSet::new(),
            cascade_data: Default::default(),
            author_styles_enabled: AuthorStylesEnabled::Yes,
            rule_tree: RuleTree::new(),
            num_rebuilds: 0,
        }
    }

    /// Returns the cascade data for the author level.
    #[inline]
    pub fn author_cascade_data(&self) -> &CascadeData {
        &self.cascade_data.author
    }

    /// Iterate through all the cascade datas from the document.
    #[inline]
    pub fn iter_origins(&self) -> DocumentCascadeDataIter {
        self.cascade_data.iter_origins()
    }

    /// Iterate over the extra data in origin order.
    #[inline]
    pub fn iter_extra_data_origins(&self) -> ExtraStyleDataIterator {
        ExtraStyleDataIterator(self.cascade_data.iter_origins())
    }

    /// Iterate over the extra data in reverse origin order.
    #[inline]
    pub fn iter_extra_data_origins_rev(&self) -> ExtraStyleDataIterator {
        ExtraStyleDataIterator(self.cascade_data.iter_origins_rev())
    }

    /// Returns the number of selectors.
    pub fn num_selectors(&self) -> usize {
        self.cascade_data
            .iter_origins()
            .map(|(d, _)| d.num_selectors)
            .sum()
    }

    /// Returns the number of declarations.
    pub fn num_declarations(&self) -> usize {
        self.cascade_data
            .iter_origins()
            .map(|(d, _)| d.num_declarations)
            .sum()
    }

    /// Returns the number of times the stylist has been rebuilt.
    pub fn num_rebuilds(&self) -> usize {
        self.num_rebuilds
    }

    /// Returns the number of revalidation_selectors.
    pub fn num_revalidation_selectors(&self) -> usize {
        self.cascade_data
            .iter_origins()
            .map(|(data, _)| data.selectors_for_cache_revalidation.len())
            .sum()
    }

    /// Returns the number of entries in invalidation maps.
    pub fn num_invalidations(&self) -> usize {
        self.cascade_data
            .iter_origins()
            .map(|(data, _)| data.invalidation_map.len())
            .sum()
    }

    /// Returns whether the given DocumentState bit is relied upon by a selector
    /// of some rule.
    pub fn has_document_state_dependency(&self, state: DocumentState) -> bool {
        self.cascade_data
            .iter_origins()
            .any(|(d, _)| d.document_state_dependencies.intersects(state))
    }

    /// Flush the list of stylesheets if they changed, ensuring the stylist is
    /// up-to-date.
    pub fn flush<E>(
        &mut self,
        guards: &StylesheetGuards,
        document_element: Option<E>,
        snapshots: Option<&SnapshotMap>,
    ) -> bool
    where
        E: TElement,
    {
        if !self.stylesheets.has_changed() {
            return false;
        }

        self.num_rebuilds += 1;

        // Update viewport_constraints regardless of which origins'
        // `CascadeData` we're updating.
        self.viewport_constraints = None;
        if viewport_rule::enabled() {
            // TODO(emilio): This doesn't look so efficient.
            //
            // Presumably when we properly implement this we can at least have a
            // bit on the stylesheet that says whether it contains viewport
            // rules to skip it entirely?
            //
            // Processing it with the rest of rules seems tricky since it
            // overrides the viewport size which may change the evaluation of
            // media queries (or may not? how are viewport units in media
            // queries defined?)
            let cascaded_rule = ViewportRule {
                declarations: viewport_rule::Cascade::from_stylesheets(
                    self.stylesheets.iter(),
                    guards,
                    &self.device,
                ).finish(),
            };

            self.viewport_constraints =
                ViewportConstraints::maybe_new(&self.device, &cascaded_rule, self.quirks_mode);

            if let Some(ref constraints) = self.viewport_constraints {
                self.device.account_for_viewport_rule(constraints);
            }
        }

        let flusher = self.stylesheets.flush(document_element, snapshots);

        let had_invalidations = flusher.had_invalidations();

        self.cascade_data
            .rebuild(&self.device, self.quirks_mode, flusher, guards)
            .unwrap_or_else(|_| warn!("OOM in Stylist::flush"));

        had_invalidations
    }

    /// Insert a given stylesheet before another stylesheet in the document.
    pub fn insert_stylesheet_before(
        &mut self,
        sheet: StylistSheet,
        before_sheet: StylistSheet,
        guard: &SharedRwLockReadGuard,
    ) {
        self.stylesheets
            .insert_stylesheet_before(Some(&self.device), sheet, before_sheet, guard)
    }

    /// Marks a given stylesheet origin as dirty, due to, for example, changes
    /// in the declarations that affect a given rule.
    ///
    /// FIXME(emilio): Eventually it'd be nice for this to become more
    /// fine-grained.
    pub fn force_stylesheet_origins_dirty(&mut self, origins: OriginSet) {
        self.stylesheets.force_dirty(origins)
    }

    /// Sets whether author style is enabled or not.
    pub fn set_author_styles_enabled(&mut self, enabled: AuthorStylesEnabled) {
        self.author_styles_enabled = enabled;
    }

    /// Returns whether we've recorded any stylesheet change so far.
    pub fn stylesheets_have_changed(&self) -> bool {
        self.stylesheets.has_changed()
    }

    /// Appends a new stylesheet to the current set.
    pub fn append_stylesheet(&mut self, sheet: StylistSheet, guard: &SharedRwLockReadGuard) {
        self.stylesheets
            .append_stylesheet(Some(&self.device), sheet, guard)
    }

    /// Appends a new stylesheet to the current set.
    pub fn prepend_stylesheet(&mut self, sheet: StylistSheet, guard: &SharedRwLockReadGuard) {
        self.stylesheets
            .prepend_stylesheet(Some(&self.device), sheet, guard)
    }

    /// Remove a given stylesheet to the current set.
    pub fn remove_stylesheet(&mut self, sheet: StylistSheet, guard: &SharedRwLockReadGuard) {
        self.stylesheets
            .remove_stylesheet(Some(&self.device), sheet, guard)
    }

    /// Returns whether for any of the applicable style rule data a given
    /// condition is true.
    pub fn any_applicable_rule_data<E, F>(&self, element: E, mut f: F) -> bool
    where
        E: TElement,
        F: FnMut(&CascadeData) -> bool,
    {
        if f(&self.cascade_data.user_agent.cascade_data) {
            return true;
        }

        let mut maybe = false;

        let doc_author_rules_apply =
            element.each_applicable_non_document_style_rule_data(|data, _, _| {
                maybe = maybe || f(&*data);
            });

        if maybe || !doc_author_rules_apply {
            return maybe;
        }

        f(&self.cascade_data.author) || f(&self.cascade_data.user)
    }

    /// Computes the style for a given "precomputed" pseudo-element, taking the
    /// universal rules and applying them.
    ///
    /// If `inherit_all` is true, then all properties are inherited from the
    /// parent; otherwise, non-inherited properties are reset to their initial
    /// values. The flow constructor uses this flag when constructing anonymous
    /// flows.
    ///
    /// TODO(emilio): The type parameter could go away with a void type
    /// implementing TElement.
    pub fn precomputed_values_for_pseudo<E>(
        &self,
        guards: &StylesheetGuards,
        pseudo: &PseudoElement,
        parent: Option<&ComputedValues>,
        font_metrics: &FontMetricsProvider,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        debug_assert!(pseudo.is_precomputed());

        let rule_node = self.rule_node_for_precomputed_pseudo(guards, pseudo, None);

        self.precomputed_values_for_pseudo_with_rule_node::<E>(
            guards,
            pseudo,
            parent,
            font_metrics,
            rule_node,
        )
    }

    /// Computes the style for a given "precomputed" pseudo-element with
    /// given rule node.
    ///
    /// TODO(emilio): The type parameter could go away with a void type
    /// implementing TElement.
    pub fn precomputed_values_for_pseudo_with_rule_node<E>(
        &self,
        guards: &StylesheetGuards,
        pseudo: &PseudoElement,
        parent: Option<&ComputedValues>,
        font_metrics: &FontMetricsProvider,
        rules: StrongRuleNode,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        self.compute_pseudo_element_style_with_inputs::<E>(
            CascadeInputs {
                rules: Some(rules),
                visited_rules: None,
            },
            pseudo,
            guards,
            parent,
            font_metrics,
            None,
        )
    }

    /// Returns the rule node for given precomputed pseudo-element.
    ///
    /// If we want to include extra declarations to this precomputed pseudo-element,
    /// we can provide a vector of ApplicableDeclarationBlock to extra_declarations
    /// argument. This is useful for providing extra @page rules.
    pub fn rule_node_for_precomputed_pseudo(
        &self,
        guards: &StylesheetGuards,
        pseudo: &PseudoElement,
        extra_declarations: Option<Vec<ApplicableDeclarationBlock>>,
    ) -> StrongRuleNode {
        let mut decl;
        let declarations = match self.cascade_data
            .user_agent
            .precomputed_pseudo_element_decls
            .get(pseudo)
        {
            Some(declarations) => match extra_declarations {
                Some(mut extra_decls) => {
                    decl = declarations.clone();
                    decl.append(&mut extra_decls);
                    Some(&decl)
                },
                None => Some(declarations),
            },
            None => extra_declarations.as_ref(),
        };

        match declarations {
            Some(decls) => self.rule_tree.insert_ordered_rules_with_important(
                decls.into_iter().map(|a| a.clone().for_rule_tree()),
                guards,
            ),
            None => self.rule_tree.root().clone(),
        }
    }

    /// Returns the style for an anonymous box of the given type.
    ///
    /// TODO(emilio): The type parameter could go away with a void type
    /// implementing TElement.
    #[cfg(feature = "servo")]
    pub fn style_for_anonymous<E>(
        &self,
        guards: &StylesheetGuards,
        pseudo: &PseudoElement,
        parent_style: &ComputedValues,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        use font_metrics::ServoMetricsProvider;
        self.precomputed_values_for_pseudo::<E>(
            guards,
            &pseudo,
            Some(parent_style),
            &ServoMetricsProvider,
        )
    }

    /// Computes a pseudo-element style lazily during layout.
    ///
    /// This can only be done for a certain set of pseudo-elements, like
    /// :selection.
    ///
    /// Check the documentation on lazy pseudo-elements in
    /// docs/components/style.md
    pub fn lazily_compute_pseudo_element_style<E>(
        &self,
        guards: &StylesheetGuards,
        element: E,
        pseudo: &PseudoElement,
        rule_inclusion: RuleInclusion,
        parent_style: &ComputedValues,
        is_probe: bool,
        font_metrics: &FontMetricsProvider,
        matching_fn: Option<&Fn(&PseudoElement) -> bool>,
    ) -> Option<Arc<ComputedValues>>
    where
        E: TElement,
    {
        let cascade_inputs = self.lazy_pseudo_rules(
            guards,
            element,
            parent_style,
            pseudo,
            is_probe,
            rule_inclusion,
            matching_fn,
        )?;

        Some(self.compute_pseudo_element_style_with_inputs(
            cascade_inputs,
            pseudo,
            guards,
            Some(parent_style),
            font_metrics,
            Some(element),
        ))
    }

    /// Computes a pseudo-element style lazily using the given CascadeInputs.
    /// This can be used for truly lazy pseudo-elements or to avoid redoing
    /// selector matching for eager pseudo-elements when we need to recompute
    /// their style with a new parent style.
    pub fn compute_pseudo_element_style_with_inputs<E>(
        &self,
        inputs: CascadeInputs,
        pseudo: &PseudoElement,
        guards: &StylesheetGuards,
        parent_style: Option<&ComputedValues>,
        font_metrics: &FontMetricsProvider,
        element: Option<E>,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        // FIXME(emilio): The lack of layout_parent_style here could be
        // worrying, but we're probably dropping the display fixup for
        // pseudos other than before and after, so it's probably ok.
        //
        // (Though the flags don't indicate so!)
        //
        // It'd be fine to assert that this isn't called with a parent style
        // where display contents is in effect, but in practice this is hard to
        // do for stuff like :-moz-fieldset-content with a
        // <fieldset style="display: contents">. That is, the computed value of
        // display for the fieldset is "contents", even though it's not the used
        // value, so we don't need to adjust in a different way anyway.
        self.cascade_style_and_visited(
            element,
            Some(pseudo),
            inputs,
            guards,
            parent_style,
            parent_style,
            parent_style,
            font_metrics,
            /* rule_cache = */ None,
            &mut RuleCacheConditions::default(),
        )
    }

    /// Computes a style using the given CascadeInputs.  This can be used to
    /// compute a style any time we know what rules apply and just need to use
    /// the given parent styles.
    ///
    /// parent_style is the style to inherit from for properties affected by
    /// first-line ancestors.
    ///
    /// parent_style_ignoring_first_line is the style to inherit from for
    /// properties not affected by first-line ancestors.
    ///
    /// layout_parent_style is the style used for some property fixups.  It's
    /// the style of the nearest ancestor with a layout box.
    ///
    /// is_link should be true if we're computing style for a link; that affects
    /// how :visited handling is done.
    pub fn cascade_style_and_visited<E>(
        &self,
        element: Option<E>,
        pseudo: Option<&PseudoElement>,
        inputs: CascadeInputs,
        guards: &StylesheetGuards,
        parent_style: Option<&ComputedValues>,
        parent_style_ignoring_first_line: Option<&ComputedValues>,
        layout_parent_style: Option<&ComputedValues>,
        font_metrics: &FontMetricsProvider,
        rule_cache: Option<&RuleCache>,
        rule_cache_conditions: &mut RuleCacheConditions,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        debug_assert!(pseudo.is_some() || element.is_some(), "Huh?");

        // We need to compute visited values if we have visited rules or if our
        // parent has visited values.
        let visited_rules = match inputs.visited_rules.as_ref() {
            Some(rules) => Some(rules),
            None => {
                if parent_style.and_then(|s| s.visited_style()).is_some() {
                    Some(inputs.rules.as_ref().unwrap_or(self.rule_tree.root()))
                } else {
                    None
                }
            }
        };

        // Read the comment on `precomputed_values_for_pseudo` to see why it's
        // difficult to assert that display: contents nodes never arrive here
        // (tl;dr: It doesn't apply for replaced elements and such, but the
        // computed value is still "contents").
        //
        // FIXME(emilio): We should assert that it holds if pseudo.is_none()!
        properties::cascade::<E>(
            &self.device,
            pseudo,
            inputs.rules.as_ref().unwrap_or(self.rule_tree.root()),
            guards,
            parent_style,
            parent_style_ignoring_first_line,
            layout_parent_style,
            visited_rules,
            font_metrics,
            self.quirks_mode,
            rule_cache,
            rule_cache_conditions,
            element,
        )
    }

    /// Computes the cascade inputs for a lazily-cascaded pseudo-element.
    ///
    /// See the documentation on lazy pseudo-elements in
    /// docs/components/style.md
    fn lazy_pseudo_rules<E>(
        &self,
        guards: &StylesheetGuards,
        element: E,
        parent_style: &ComputedValues,
        pseudo: &PseudoElement,
        is_probe: bool,
        rule_inclusion: RuleInclusion,
        matching_fn: Option<&Fn(&PseudoElement) -> bool>,
    ) -> Option<CascadeInputs>
    where
        E: TElement,
    {
        let pseudo = pseudo.canonical();
        debug_assert!(pseudo.is_lazy());

        // Apply the selector flags. We should be in sequential mode
        // already, so we can directly apply the parent flags.
        let mut set_selector_flags = |element: &E, flags: ElementSelectorFlags| {
            if cfg!(feature = "servo") {
                // Servo calls this function from the worker, but only for internal
                // pseudos, so we should never generate selector flags here.
                unreachable!("internal pseudo generated slow selector flags?");
            }

            // No need to bother setting the selector flags when we're computing
            // default styles.
            if rule_inclusion == RuleInclusion::DefaultOnly {
                return;
            }

            // Gecko calls this from sequential mode, so we can directly apply
            // the flags.
            debug_assert_eq!(thread_state::get(), ThreadState::LAYOUT);
            let self_flags = flags.for_self();
            if !self_flags.is_empty() {
                unsafe {
                    element.set_selector_flags(self_flags);
                }
            }
            let parent_flags = flags.for_parent();
            if !parent_flags.is_empty() {
                if let Some(p) = element.parent_element() {
                    unsafe {
                        p.set_selector_flags(parent_flags);
                    }
                }
            }
        };

        let mut declarations = ApplicableDeclarationList::new();
        let mut matching_context = MatchingContext::new(
            MatchingMode::ForStatelessPseudoElement,
            None,
            None,
            self.quirks_mode,
        );

        matching_context.pseudo_element_matching_fn = matching_fn;

        self.push_applicable_declarations(
            element,
            Some(&pseudo),
            None,
            None,
            AnimationRules(None, None),
            rule_inclusion,
            &mut declarations,
            &mut matching_context,
            &mut set_selector_flags,
        );

        if declarations.is_empty() && is_probe {
            return None;
        }

        let rules = self.rule_tree.compute_rule_node(&mut declarations, guards);

        let mut visited_rules = None;
        if parent_style.visited_style().is_some() {
            let mut declarations = ApplicableDeclarationList::new();
            let mut matching_context = MatchingContext::new_for_visited(
                MatchingMode::ForStatelessPseudoElement,
                None,
                None,
                VisitedHandlingMode::RelevantLinkVisited,
                self.quirks_mode,
            );
            matching_context.pseudo_element_matching_fn = matching_fn;

            self.push_applicable_declarations(
                element,
                Some(&pseudo),
                None,
                None,
                AnimationRules(None, None),
                rule_inclusion,
                &mut declarations,
                &mut matching_context,
                &mut set_selector_flags,
            );
            if !declarations.is_empty() {
                let rule_node = self.rule_tree.insert_ordered_rules_with_important(
                    declarations.drain().map(|a| a.for_rule_tree()),
                    guards,
                );
                if rule_node != *self.rule_tree.root() {
                    visited_rules = Some(rule_node);
                }
            }
        }

        Some(CascadeInputs {
            rules: Some(rules),
            visited_rules,
        })
    }

    /// Set a given device, which may change the styles that apply to the
    /// document.
    ///
    /// Returns the sheet origins that were actually affected.
    ///
    /// This means that we may need to rebuild style data even if the
    /// stylesheets haven't changed.
    ///
    /// Also, the device that arrives here may need to take the viewport rules
    /// into account.
    pub fn set_device(&mut self, mut device: Device, guards: &StylesheetGuards) -> OriginSet {
        if viewport_rule::enabled() {
            let cascaded_rule = {
                let stylesheets = self.stylesheets.iter();

                ViewportRule {
                    declarations: viewport_rule::Cascade::from_stylesheets(
                        stylesheets.clone(),
                        guards,
                        &device,
                    ).finish(),
                }
            };

            self.viewport_constraints =
                ViewportConstraints::maybe_new(&device, &cascaded_rule, self.quirks_mode);

            if let Some(ref constraints) = self.viewport_constraints {
                device.account_for_viewport_rule(constraints);
            }
        }

        self.device = device;
        self.media_features_change_changed_style(guards, &self.device)
    }

    /// Returns whether, given a media feature change, any previously-applicable
    /// style has become non-applicable, or vice-versa for each origin, using
    /// `device`.
    ///
    /// Passing `device` is needed because this is used for XBL in Gecko, which
    /// can be stale in various ways, so we need to pass the device of the
    /// document itself, which is what is kept up-to-date.
    ///
    /// Arguably XBL should use something more lightweight than a Stylist.
    pub fn media_features_change_changed_style(
        &self,
        guards: &StylesheetGuards,
        device: &Device,
    ) -> OriginSet {
        debug!("Stylist::media_features_change_changed_style {:?}", device);

        let mut origins = OriginSet::empty();
        let stylesheets = self.stylesheets.iter();

        for (stylesheet, origin) in stylesheets {
            if origins.contains(origin.into()) {
                continue;
            }

            let guard = guards.for_origin(origin);
            let origin_cascade_data = self.cascade_data.borrow_for_origin(origin);

            let affected_changed = !origin_cascade_data.media_feature_affected_matches(
                stylesheet,
                guard,
                device,
                self.quirks_mode,
            );

            if affected_changed {
                origins |= origin;
            }
        }

        origins
    }

    /// Returns the viewport constraints that apply to this document because of
    /// a @viewport rule.
    pub fn viewport_constraints(&self) -> Option<&ViewportConstraints> {
        self.viewport_constraints.as_ref()
    }

    /// Returns the Quirks Mode of the document.
    pub fn quirks_mode(&self) -> QuirksMode {
        self.quirks_mode
    }

    /// Sets the quirks mode of the document.
    pub fn set_quirks_mode(&mut self, quirks_mode: QuirksMode) {
        if self.quirks_mode == quirks_mode {
            return;
        }
        self.quirks_mode = quirks_mode;
        self.force_stylesheet_origins_dirty(OriginSet::all());
    }

    /// Returns the applicable CSS declarations for the given element.
    ///
    /// This corresponds to `ElementRuleCollector` in WebKit, and should push to
    /// elements in the list in the order defined by:
    ///
    /// https://drafts.csswg.org/css-cascade/#cascade-origin
    pub fn push_applicable_declarations<E, F>(
        &self,
        element: E,
        pseudo_element: Option<&PseudoElement>,
        style_attribute: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
        smil_override: Option<ArcBorrow<Locked<PropertyDeclarationBlock>>>,
        animation_rules: AnimationRules,
        rule_inclusion: RuleInclusion,
        applicable_declarations: &mut ApplicableDeclarationList,
        context: &mut MatchingContext<E::Impl>,
        flags_setter: &mut F,
    ) where
        E: TElement,
        F: FnMut(&E, ElementSelectorFlags),
    {
        // Gecko definitely has pseudo-elements with style attributes, like
        // ::-moz-color-swatch.
        debug_assert!(
            cfg!(feature = "gecko") || style_attribute.is_none() || pseudo_element.is_none(),
            "Style attributes do not apply to pseudo-elements"
        );
        debug_assert!(pseudo_element.map_or(true, |p| !p.is_precomputed()));

        let rule_hash_target = element.rule_hash_target();

        debug!(
            "Determining if style is shareable: pseudo: {}",
            pseudo_element.is_some()
        );

        let matches_user_rules = rule_hash_target.matches_user_and_author_rules();
        let matches_author_rules =
            matches_user_rules && self.author_styles_enabled == AuthorStylesEnabled::Yes;

        // Normal user-agent rules.
        if let Some(map) = self.cascade_data
            .user_agent
            .cascade_data
            .normal_rules(pseudo_element)
        {
            map.get_all_matching_rules(
                element,
                rule_hash_target,
                applicable_declarations,
                context,
                flags_setter,
                CascadeLevel::UANormal,
                0,
            );
        }

        // NB: the following condition, although it may look somewhat
        // inaccurate, would be equivalent to something like:
        //
        //     element.matches_user_and_author_rules() ||
        //     (is_implemented_pseudo &&
        //      rule_hash_target.matches_user_and_author_rules())
        //
        // Which may be more what you would probably expect.
        if matches_user_rules {
            // User normal rules.
            if let Some(map) = self.cascade_data.user.normal_rules(pseudo_element) {
                map.get_all_matching_rules(
                    element,
                    rule_hash_target,
                    applicable_declarations,
                    context,
                    flags_setter,
                    CascadeLevel::UserNormal,
                    0,
                );
            }
        }

        if rule_inclusion == RuleInclusion::DefaultOnly {
            return;
        }

        if pseudo_element.is_none() {
            // Presentational hints.
            //
            // These go before author rules, but after user rules, see:
            // https://drafts.csswg.org/css-cascade/#preshint
            let length_before_preshints = applicable_declarations.len();
            element.synthesize_presentational_hints_for_legacy_attributes(
                context.visited_handling(),
                applicable_declarations,
            );
            if cfg!(debug_assertions) {
                if applicable_declarations.len() != length_before_preshints {
                    for declaration in &applicable_declarations[length_before_preshints..] {
                        assert_eq!(declaration.level(), CascadeLevel::PresHints);
                    }
                }
            }
        }

        let mut match_document_author_rules = matches_author_rules;
        let mut shadow_cascade_order = 0;

        // XBL / Shadow DOM rules, which are author rules too.
        //
        // TODO(emilio): Cascade order here is wrong for Shadow DOM. In
        // particular, normally document rules override ::slotted() rules, but
        // for !important it should be the other way around. So probably we need
        // to add some sort of AuthorScoped cascade level or something.
        if matches_author_rules {
            if let Some(shadow) = rule_hash_target.shadow_root() {
                if let Some(map) = shadow.style_data().host_rules(pseudo_element) {
                    context.with_shadow_host(Some(rule_hash_target), |context| {
                        map.get_all_matching_rules(
                            element,
                            rule_hash_target,
                            applicable_declarations,
                            context,
                            flags_setter,
                            CascadeLevel::InnerShadowNormal,
                            shadow_cascade_order,
                        );
                    });
                    shadow_cascade_order += 1;
                }
            }

            // Match slotted rules in reverse order, so that the outer slotted
            // rules come before the inner rules (and thus have less priority).
            let mut slots = SmallVec::<[_; 3]>::new();
            let mut current = rule_hash_target.assigned_slot();
            while let Some(slot) = current {
                slots.push(slot);
                current = slot.assigned_slot();
            }

            for slot in slots.iter().rev() {
                let shadow = slot.containing_shadow().unwrap();
                let styles = shadow.style_data();
                if let Some(map) = styles.slotted_rules(pseudo_element) {
                    context.with_shadow_host(Some(shadow.host()), |context| {
                        map.get_all_matching_rules(
                            element,
                            rule_hash_target,
                            applicable_declarations,
                            context,
                            flags_setter,
                            CascadeLevel::InnerShadowNormal,
                            shadow_cascade_order,
                        );
                    });
                    shadow_cascade_order += 1;
                }
            }

            if let Some(containing_shadow) = rule_hash_target.containing_shadow() {
                let cascade_data = containing_shadow.style_data();
                let host = containing_shadow.host();
                if let Some(map) = cascade_data.normal_rules(pseudo_element) {
                    context.with_shadow_host(Some(host), |context| {
                        map.get_all_matching_rules(
                            element,
                            rule_hash_target,
                            applicable_declarations,
                            context,
                            flags_setter,
                            CascadeLevel::SameTreeAuthorNormal,
                            shadow_cascade_order,
                        );
                    });
                    shadow_cascade_order += 1;
                }

                // NOTE(emilio): Hack so <svg:use> matches document rules as
                // expected.
                //
                // This is not a problem for invalidation and that kind of stuff
                // because they still don't match rules based on elements
                // outside of the shadow tree, and because the <svg:use> subtree
                // is immutable and recreated each time the source tree changes.
                //
                // See: https://github.com/w3c/svgwg/issues/504
                //
                // Note that we always resolve URLs against the document, so we
                // can't get into a nested shadow situation here.
                //
                // See: https://github.com/w3c/svgwg/issues/505
                //
                let host_is_svg_use =
                    host.is_svg_element() &&
                    host.local_name() == &*local_name!("use");

                match_document_author_rules = host_is_svg_use;
            }
        }

        // FIXME(emilio): This doesn't account for the author_styles_enabled
        // stuff...
        let cut_xbl_binding_inheritance =
            element.each_xbl_cascade_data(|cascade_data, quirks_mode| {
                if let Some(map) = cascade_data.normal_rules(pseudo_element) {
                    // NOTE(emilio): This is needed because the XBL stylist may
                    // think it has a different quirks mode than the document.
                    let mut matching_context = MatchingContext::new(
                        context.matching_mode(),
                        context.bloom_filter,
                        context.nth_index_cache.as_mut().map(|s| &mut **s),
                        quirks_mode,
                    );
                    matching_context.pseudo_element_matching_fn =
                        context.pseudo_element_matching_fn;

                    // SameTreeAuthorNormal instead of InnerShadowNormal to
                    // preserve behavior, though that's kinda fishy...
                    map.get_all_matching_rules(
                        element,
                        rule_hash_target,
                        applicable_declarations,
                        &mut matching_context,
                        flags_setter,
                        CascadeLevel::SameTreeAuthorNormal,
                        shadow_cascade_order,
                    );
                }
            });

        match_document_author_rules &= !cut_xbl_binding_inheritance;

        if match_document_author_rules {
            // Author normal rules.
            if let Some(map) = self.cascade_data.author.normal_rules(pseudo_element) {
                map.get_all_matching_rules(
                    element,
                    rule_hash_target,
                    applicable_declarations,
                    context,
                    flags_setter,
                    CascadeLevel::SameTreeAuthorNormal,
                    shadow_cascade_order,
                );
            }
        }

        // Style attribute ("Normal override declarations").
        if let Some(sa) = style_attribute {
            applicable_declarations.push(ApplicableDeclarationBlock::from_declarations(
                sa.clone_arc(),
                CascadeLevel::StyleAttributeNormal,
            ));
        }

        // Declarations from SVG SMIL animation elements.
        if let Some(so) = smil_override {
            applicable_declarations.push(ApplicableDeclarationBlock::from_declarations(
                so.clone_arc(),
                CascadeLevel::SMILOverride,
            ));
        }

        // The animations sheet (CSS animations, script-generated
        // animations, and CSS transitions that are no longer tied to CSS
        // markup).
        if let Some(anim) = animation_rules.0 {
            applicable_declarations.push(ApplicableDeclarationBlock::from_declarations(
                anim.clone(),
                CascadeLevel::Animations,
            ));
        }

        //
        // !important rules are handled during rule tree insertion.
        //

        // The transitions sheet (CSS transitions that are tied to CSS
        // markup).
        if let Some(anim) = animation_rules.1 {
            applicable_declarations.push(ApplicableDeclarationBlock::from_declarations(
                anim.clone(),
                CascadeLevel::Transitions,
            ));
        }
    }

    /// Given an id, returns whether there might be any rules for that id in any
    /// of our rule maps.
    #[inline]
    pub fn may_have_rules_for_id<E>(&self, id: &WeakAtom, element: E) -> bool
    where
        E: TElement,
    {
        // If id needs to be compared case-insensitively, the logic below
        // wouldn't work. Just conservatively assume it may have such rules.
        match self.quirks_mode().classes_and_ids_case_sensitivity() {
            CaseSensitivity::AsciiCaseInsensitive => return true,
            CaseSensitivity::CaseSensitive => {},
        }

        let hash = id.get_hash();
        self.any_applicable_rule_data(element, |data| data.mapped_ids.might_contain_hash(hash))
    }

    /// Returns the registered `@keyframes` animation for the specified name.
    #[inline]
    pub fn get_animation<'a, E>(
        &'a self,
        name: &Atom,
        element: E,
    ) -> Option<&'a KeyframesAnimation>
    where
        E: TElement + 'a,
    {
        macro_rules! try_find_in {
            ($data:expr) => {
                if let Some(animation) = $data.animations.get(name) {
                    return Some(animation);
                }
            }
        }

        // NOTE(emilio): We implement basically what Blink does for this case,
        // which is [1] as of this writing.
        //
        // See [2] for the spec discussion about what to do about this. WebKit's
        // behavior makes a bit more sense off-hand, but it's way more complex
        // to implement, and it makes value computation having to thread around
        // the cascade level, which is not great. Also, it breaks if you inherit
        // animation-name from an element in a different tree.
        //
        // See [3] for the bug to implement whatever gets resolved, and related
        // bugs for a bit more context.
        //
        // [1]: https://cs.chromium.org/chromium/src/third_party/blink/renderer/
        //        core/css/resolver/style_resolver.cc?l=1267&rcl=90f9f8680ebb4a87d177f3b0833372ae4e0c88d8
        // [2]: https://github.com/w3c/csswg-drafts/issues/1995
        // [3]: https://bugzil.la/1458189
        if let Some(shadow) = element.shadow_root() {
            try_find_in!(shadow.style_data());
        }

        if let Some(shadow) = element.containing_shadow() {
            try_find_in!(shadow.style_data());
        } else {
            try_find_in!(self.cascade_data.author);
        }

        try_find_in!(self.cascade_data.user);
        try_find_in!(self.cascade_data.user_agent.cascade_data);

        None
    }

    /// Computes the match results of a given element against the set of
    /// revalidation selectors.
    pub fn match_revalidation_selectors<E, F>(
        &self,
        element: E,
        bloom: Option<&BloomFilter>,
        nth_index_cache: &mut NthIndexCache,
        flags_setter: &mut F,
    ) -> SmallBitVec
    where
        E: TElement,
        F: FnMut(&E, ElementSelectorFlags),
    {
        // NB: `MatchingMode` doesn't really matter, given we don't share style
        // between pseudos.
        let mut matching_context = MatchingContext::new(
            MatchingMode::Normal,
            bloom,
            Some(nth_index_cache),
            self.quirks_mode,
        );

        // Note that, by the time we're revalidating, we're guaranteed that the
        // candidate and the entry have the same id, classes, and local name.
        // This means we're guaranteed to get the same rulehash buckets for all
        // the lookups, which means that the bitvecs are comparable. We verify
        // this in the caller by asserting that the bitvecs are same-length.
        let mut results = SmallBitVec::new();
        for (data, _) in self.cascade_data.iter_origins() {
            data.selectors_for_cache_revalidation.lookup(
                element,
                self.quirks_mode,
                |selector_and_hashes| {
                    results.push(matches_selector(
                        &selector_and_hashes.selector,
                        selector_and_hashes.selector_offset,
                        Some(&selector_and_hashes.hashes),
                        &element,
                        &mut matching_context,
                        flags_setter,
                    ));
                    true
                },
            );
        }

        element.each_applicable_non_document_style_rule_data(|data, quirks_mode, host| {
            matching_context.with_shadow_host(host, |matching_context| {
                data.selectors_for_cache_revalidation.lookup(
                    element,
                    quirks_mode,
                    |selector_and_hashes| {
                        results.push(matches_selector(
                            &selector_and_hashes.selector,
                            selector_and_hashes.selector_offset,
                            Some(&selector_and_hashes.hashes),
                            &element,
                            matching_context,
                            flags_setter,
                        ));
                        true
                    },
                );
            })
        });

        results
    }

    /// Computes styles for a given declaration with parent_style.
    ///
    /// FIXME(emilio): the lack of pseudo / cascade flags look quite dubious,
    /// hopefully this is only used for some canvas font stuff.
    ///
    /// TODO(emilio): The type parameter can go away when
    /// https://github.com/rust-lang/rust/issues/35121 is fixed.
    pub fn compute_for_declarations<E>(
        &self,
        guards: &StylesheetGuards,
        parent_style: &ComputedValues,
        declarations: Arc<Locked<PropertyDeclarationBlock>>,
    ) -> Arc<ComputedValues>
    where
        E: TElement,
    {
        use font_metrics::get_metrics_provider_for_product;

        let block = declarations.read_with(guards.author);
        let iter_declarations = || {
            block.declaration_importance_iter().map(|(declaration, importance)| {
                debug_assert!(!importance.important());
                (declaration, CascadeLevel::StyleAttributeNormal)
            })
        };

        let metrics = get_metrics_provider_for_product();

        // We don't bother inserting these declarations in the rule tree, since
        // it'd be quite useless and slow.
        properties::apply_declarations::<E, _, _>(
            &self.device,
            /* pseudo = */ None,
            self.rule_tree.root(),
            guards,
            iter_declarations,
            Some(parent_style),
            Some(parent_style),
            Some(parent_style),
            &metrics,
            CascadeMode::Unvisited { visited_rules: None },
            self.quirks_mode,
            /* rule_cache = */ None,
            &mut Default::default(),
            /* element = */ None,
        )
    }

    /// Accessor for a shared reference to the device.
    #[inline]
    pub fn device(&self) -> &Device {
        &self.device
    }

    /// Accessor for a mutable reference to the device.
    #[inline]
    pub fn device_mut(&mut self) -> &mut Device {
        &mut self.device
    }

    /// Accessor for a shared reference to the rule tree.
    #[inline]
    pub fn rule_tree(&self) -> &RuleTree {
        &self.rule_tree
    }

    /// Measures heap usage.
    #[cfg(feature = "gecko")]
    pub fn add_size_of(&self, ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
        self.cascade_data.add_size_of(ops, sizes);
        sizes.mRuleTree += self.rule_tree.size_of(ops);

        // We may measure other fields in the future if DMD says it's worth it.
    }

    /// Shutdown the static data that this module stores.
    pub fn shutdown() {
        UA_CASCADE_DATA_CACHE.lock().unwrap().clear()
    }
}

/// This struct holds data which users of Stylist may want to extract
/// from stylesheets which can be done at the same time as updating.
#[derive(Debug, Default)]
#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
pub struct ExtraStyleData {
    /// A list of effective font-face rules and their origin.
    #[cfg(feature = "gecko")]
    pub font_faces: Vec<Arc<Locked<FontFaceRule>>>,

    /// A list of effective font-feature-values rules.
    #[cfg(feature = "gecko")]
    pub font_feature_values: Vec<Arc<Locked<FontFeatureValuesRule>>>,

    /// A map of effective counter-style rules.
    #[cfg(feature = "gecko")]
    pub counter_styles: PrecomputedHashMap<Atom, Arc<Locked<CounterStyleRule>>>,

    /// A map of effective page rules.
    #[cfg(feature = "gecko")]
    pub pages: Vec<Arc<Locked<PageRule>>>,
}

#[cfg(feature = "gecko")]
unsafe impl Sync for ExtraStyleData {}
#[cfg(feature = "gecko")]
unsafe impl Send for ExtraStyleData {}

#[cfg(feature = "gecko")]
impl ExtraStyleData {
    /// Add the given @font-face rule.
    fn add_font_face(&mut self, rule: &Arc<Locked<FontFaceRule>>) {
        self.font_faces.push(rule.clone());
    }

    /// Add the given @font-feature-values rule.
    fn add_font_feature_values(&mut self, rule: &Arc<Locked<FontFeatureValuesRule>>) {
        self.font_feature_values.push(rule.clone());
    }

    /// Add the given @counter-style rule.
    fn add_counter_style(
        &mut self,
        guard: &SharedRwLockReadGuard,
        rule: &Arc<Locked<CounterStyleRule>>,
    ) {
        let name = rule.read_with(guard).name().0.clone();
        self.counter_styles.insert(name, rule.clone());
    }

    /// Add the given @page rule.
    fn add_page(&mut self, rule: &Arc<Locked<PageRule>>) {
        self.pages.push(rule.clone());
    }
}

impl ExtraStyleData {
    fn clear(&mut self) {
        #[cfg(feature = "gecko")]
        {
            self.font_faces.clear();
            self.font_feature_values.clear();
            self.counter_styles.clear();
            self.pages.clear();
        }
    }
}

/// An iterator over the different ExtraStyleData.
pub struct ExtraStyleDataIterator<'a>(DocumentCascadeDataIter<'a>);

impl<'a> Iterator for ExtraStyleDataIterator<'a> {
    type Item = (&'a ExtraStyleData, Origin);

    fn next(&mut self) -> Option<Self::Item> {
        self.0.next().map(|d| (&d.0.extra_data, d.1))
    }
}

#[cfg(feature = "gecko")]
impl MallocSizeOf for ExtraStyleData {
    /// Measure heap usage.
    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        let mut n = 0;
        n += self.font_faces.shallow_size_of(ops);
        n += self.font_feature_values.shallow_size_of(ops);
        n += self.counter_styles.shallow_size_of(ops);
        n += self.pages.shallow_size_of(ops);
        n
    }
}

/// SelectorMapEntry implementation for use in our revalidation selector map.
#[cfg_attr(feature = "gecko", derive(MallocSizeOf))]
#[derive(Clone, Debug)]
struct RevalidationSelectorAndHashes {
    #[cfg_attr(feature = "gecko",
               ignore_malloc_size_of = "CssRules have primary refs, we measure there")]
    selector: Selector<SelectorImpl>,
    selector_offset: usize,
    hashes: AncestorHashes,
}

impl RevalidationSelectorAndHashes {
    fn new(selector: Selector<SelectorImpl>, hashes: AncestorHashes) -> Self {
        let selector_offset = {
            // We basically want to check whether the first combinator is a
            // pseudo-element combinator.  If it is, we want to use the offset
            // one past it.  Otherwise, our offset is 0.
            let mut index = 0;
            let mut iter = selector.iter();

            // First skip over the first ComplexSelector.
            //
            // We can't check what sort of what combinator we have until we do
            // that.
            for _ in &mut iter {
                index += 1; // Simple selector
            }

            match iter.next_sequence() {
                Some(Combinator::PseudoElement) => index + 1, // +1 for the combinator
                _ => 0,
            }
        };

        RevalidationSelectorAndHashes {
            selector,
            selector_offset,
            hashes,
        }
    }
}

impl SelectorMapEntry for RevalidationSelectorAndHashes {
    fn selector(&self) -> SelectorIter<SelectorImpl> {
        self.selector.iter_from(self.selector_offset)
    }
}

/// A selector visitor implementation that collects all the state the Stylist
/// cares about a selector.
struct StylistSelectorVisitor<'a> {
    /// Whether the selector needs revalidation for the style sharing cache.
    needs_revalidation: bool,
    /// Whether we've past the rightmost compound selector, not counting
    /// pseudo-elements.
    passed_rightmost_selector: bool,
    /// The filter with all the id's getting referenced from rightmost
    /// selectors.
    mapped_ids: &'a mut NonCountingBloomFilter,
    /// The filter with the local names of attributes there are selectors for.
    attribute_dependencies: &'a mut NonCountingBloomFilter,
    /// Whether there's any attribute selector for the [style] attribute.
    style_attribute_dependency: &'a mut bool,
    /// All the states selectors in the page reference.
    state_dependencies: &'a mut ElementState,
    /// All the document states selectors in the page reference.
    document_state_dependencies: &'a mut DocumentState,
}

fn component_needs_revalidation(
    c: &Component<SelectorImpl>,
    passed_rightmost_selector: bool,
) -> bool {
    match *c {
        Component::ID(_) => {
            // TODO(emilio): This could also check that the ID is not already in
            // the rule hash. In that case, we could avoid making this a
            // revalidation selector too.
            //
            // See https://bugzilla.mozilla.org/show_bug.cgi?id=1369611
            passed_rightmost_selector
        },
        Component::AttributeInNoNamespaceExists { .. } |
        Component::AttributeInNoNamespace { .. } |
        Component::AttributeOther(_) |
        Component::Empty |
        Component::FirstChild |
        Component::LastChild |
        Component::OnlyChild |
        Component::NthChild(..) |
        Component::NthLastChild(..) |
        Component::NthOfType(..) |
        Component::NthLastOfType(..) |
        Component::FirstOfType |
        Component::LastOfType |
        Component::OnlyOfType => true,
        Component::NonTSPseudoClass(ref p) => p.needs_cache_revalidation(),
        _ => false,
    }
}

impl<'a> SelectorVisitor for StylistSelectorVisitor<'a> {
    type Impl = SelectorImpl;

    fn visit_complex_selector(&mut self, combinator: Option<Combinator>) -> bool {
        self.needs_revalidation =
            self.needs_revalidation || combinator.map_or(false, |c| c.is_sibling());

        // NOTE(emilio): This works properly right now because we can't store
        // complex selectors in nested selectors, otherwise we may need to
        // rethink this.
        //
        // Also, note that this call happens before we visit any of the simple
        // selectors in the next ComplexSelector, so we can use this to skip
        // looking at them.
        self.passed_rightmost_selector = self.passed_rightmost_selector ||
            !matches!(combinator, None | Some(Combinator::PseudoElement));

        true
    }

    fn visit_attribute_selector(
        &mut self,
        _ns: &NamespaceConstraint<&Namespace>,
        name: &LocalName,
        lower_name: &LocalName,
    ) -> bool {
        if *lower_name == local_name!("style") {
            *self.style_attribute_dependency = true;
        } else {
            self.attribute_dependencies.insert_hash(name.get_hash());
            self.attribute_dependencies
                .insert_hash(lower_name.get_hash());
        }
        true
    }

    fn visit_simple_selector(&mut self, s: &Component<SelectorImpl>) -> bool {
        self.needs_revalidation = self.needs_revalidation ||
            component_needs_revalidation(s, self.passed_rightmost_selector);

        match *s {
            Component::NonTSPseudoClass(ref p) => {
                self.state_dependencies.insert(p.state_flag());
                self.document_state_dependencies
                    .insert(p.document_state_flag());
            },
            Component::ID(ref id) if !self.passed_rightmost_selector => {
                // We want to stop storing mapped ids as soon as we've moved off
                // the rightmost ComplexSelector that is not a pseudo-element.
                //
                // That can be detected by a visit_complex_selector call with a
                // combinator other than None and PseudoElement.
                //
                // Importantly, this call happens before we visit any of the
                // simple selectors in that ComplexSelector.
                //
                // NOTE(emilio): See the comment regarding on when this may
                // break in visit_complex_selector.
                self.mapped_ids.insert_hash(id.get_hash());
            },
            _ => {},
        }

        true
    }
}

/// A set of rules for element and pseudo-elements.
#[derive(Debug, Default, MallocSizeOf)]
struct ElementAndPseudoRules {
    /// Rules from stylesheets at this `CascadeData`'s origin.
    element_map: SelectorMap<Rule>,

    /// Rules from stylesheets at this `CascadeData`'s origin that correspond
    /// to a given pseudo-element.
    ///
    /// FIXME(emilio): There are a bunch of wasted entries here in practice.
    /// Figure out a good way to do a `PerNonAnonBox` and `PerAnonBox` (for
    /// `precomputed_values_for_pseudo`) without duplicating a lot of code.
    pseudos_map: PerPseudoElementMap<Box<SelectorMap<Rule>>>,
}

impl ElementAndPseudoRules {
    #[inline(always)]
    fn insert(
        &mut self,
        rule: Rule,
        pseudo_element: Option<&PseudoElement>,
        quirks_mode: QuirksMode,
    ) -> Result<(), FailedAllocationError> {
        debug_assert!(pseudo_element.map_or(true, |pseudo| !pseudo.is_precomputed()));

        let map = match pseudo_element {
            None => &mut self.element_map,
            Some(pseudo) => self.pseudos_map
                .get_or_insert_with(&pseudo.canonical(), || Box::new(SelectorMap::new())),
        };

        map.insert(rule, quirks_mode)
    }

    fn clear(&mut self) {
        self.element_map.clear();
        self.pseudos_map.clear();
    }

    #[inline]
    fn rules(&self, pseudo: Option<&PseudoElement>) -> Option<&SelectorMap<Rule>> {
        match pseudo {
            Some(pseudo) => self.pseudos_map.get(&pseudo.canonical()).map(|p| &**p),
            None => Some(&self.element_map),
        }
    }

    /// Measures heap usage.
    #[cfg(feature = "gecko")]
    fn add_size_of(&self, ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
        sizes.mElementAndPseudosMaps += self.element_map.size_of(ops);

        for elem in self.pseudos_map.iter() {
            if let Some(ref elem) = *elem {
                sizes.mElementAndPseudosMaps += <Box<_> as MallocSizeOf>::size_of(elem, ops);
            }
        }
    }
}

/// Data resulting from performing the CSS cascade that is specific to a given
/// origin.
///
/// FIXME(emilio): Consider renaming and splitting in `CascadeData` and
/// `InvalidationData`? That'd make `clear_cascade_data()` clearer.
#[derive(Debug, MallocSizeOf)]
pub struct CascadeData {
    /// The data coming from normal style rules that apply to elements at this
    /// cascade level.
    normal_rules: ElementAndPseudoRules,

    /// The `:host` pseudo rules that are the rightmost selector (without
    /// accounting for pseudo-elements).
    host_rules: Option<Box<ElementAndPseudoRules>>,

    /// The data coming from ::slotted() pseudo-element rules.
    ///
    /// We need to store them separately because an element needs to match
    /// ::slotted() pseudo-element rules in different shadow roots.
    ///
    /// In particular, we need to go through all the style data in all the
    /// containing style scopes starting from the closest assigned slot.
    slotted_rules: Option<Box<ElementAndPseudoRules>>,

    /// The invalidation map for these rules.
    invalidation_map: InvalidationMap,

    /// The attribute local names that appear in attribute selectors.  Used
    /// to avoid taking element snapshots when an irrelevant attribute changes.
    /// (We don't bother storing the namespace, since namespaced attributes
    /// are rare.)
    #[ignore_malloc_size_of = "just an array"]
    attribute_dependencies: NonCountingBloomFilter,

    /// Whether `"style"` appears in an attribute selector.  This is not common,
    /// and by tracking this explicitly, we can avoid taking an element snapshot
    /// in the common case of style=""` changing due to modifying
    /// `element.style`.  (We could track this in `attribute_dependencies`, like
    /// all other attributes, but we should probably not risk incorrectly
    /// returning `true` for `"style"` just due to a hash collision.)
    style_attribute_dependency: bool,

    /// The element state bits that are relied on by selectors.  Like
    /// `attribute_dependencies`, this is used to avoid taking element snapshots
    /// when an irrelevant element state bit changes.
    state_dependencies: ElementState,

    /// The document state bits that are relied on by selectors.  This is used
    /// to tell whether we need to restyle the entire document when a document
    /// state bit changes.
    document_state_dependencies: DocumentState,

    /// The ids that appear in the rightmost complex selector of selectors (and
    /// hence in our selector maps).  Used to determine when sharing styles is
    /// safe: we disallow style sharing for elements whose id matches this
    /// filter, and hence might be in one of our selector maps.
    #[ignore_malloc_size_of = "just an array"]
    mapped_ids: NonCountingBloomFilter,

    /// Selectors that require explicit cache revalidation (i.e. which depend
    /// on state that is not otherwise visible to the cache, like attributes or
    /// tree-structural state like child index and pseudos).
    #[ignore_malloc_size_of = "Arc"]
    selectors_for_cache_revalidation: SelectorMap<RevalidationSelectorAndHashes>,

    /// A map with all the animations at this `CascadeData`'s origin, indexed
    /// by name.
    animations: PrecomputedHashMap<Atom, KeyframesAnimation>,

    /// Effective media query results cached from the last rebuild.
    effective_media_query_results: EffectiveMediaQueryResults,

    /// Extra data, like different kinds of rules, etc.
    extra_data: ExtraStyleData,

    /// A monotonically increasing counter to represent the order on which a
    /// style rule appears in a stylesheet, needed to sort them by source order.
    rules_source_order: u32,

    /// The total number of selectors.
    num_selectors: usize,

    /// The total number of declarations.
    num_declarations: usize,
}

impl CascadeData {
    /// Creates an empty `CascadeData`.
    pub fn new() -> Self {
        Self {
            normal_rules: ElementAndPseudoRules::default(),
            host_rules: None,
            slotted_rules: None,
            invalidation_map: InvalidationMap::new(),
            attribute_dependencies: NonCountingBloomFilter::new(),
            style_attribute_dependency: false,
            state_dependencies: ElementState::empty(),
            document_state_dependencies: DocumentState::empty(),
            mapped_ids: NonCountingBloomFilter::new(),
            selectors_for_cache_revalidation: SelectorMap::new(),
            animations: Default::default(),
            extra_data: ExtraStyleData::default(),
            effective_media_query_results: EffectiveMediaQueryResults::new(),
            rules_source_order: 0,
            num_selectors: 0,
            num_declarations: 0,
        }
    }

    /// Rebuild the cascade data from a given SheetCollection, incrementally if
    /// possible.
    pub fn rebuild<'a, S>(
        &mut self,
        device: &Device,
        quirks_mode: QuirksMode,
        collection: SheetCollectionFlusher<S>,
        guard: &SharedRwLockReadGuard,
    ) -> Result<(), FailedAllocationError>
    where
        S: StylesheetInDocument + ToMediaListKey + PartialEq + 'static,
    {
        if !collection.dirty() {
            return Ok(());
        }

        let validity = collection.data_validity();

        match validity {
            DataValidity::Valid => {},
            DataValidity::CascadeInvalid => self.clear_cascade_data(),
            DataValidity::FullyInvalid => self.clear(),
        }

        for (stylesheet, rebuild_kind) in collection {
            self.add_stylesheet(
                device,
                quirks_mode,
                stylesheet,
                guard,
                rebuild_kind,
                /* precomputed_pseudo_element_decls = */ None,
            )?;
        }

        Ok(())
    }

    /// Returns the invalidation map.
    pub fn invalidation_map(&self) -> &InvalidationMap {
        &self.invalidation_map
    }

    /// Returns whether the given ElementState bit is relied upon by a selector
    /// of some rule.
    #[inline]
    pub fn has_state_dependency(&self, state: ElementState) -> bool {
        self.state_dependencies.intersects(state)
    }

    /// Returns whether the given attribute might appear in an attribute
    /// selector of some rule.
    #[inline]
    pub fn might_have_attribute_dependency(&self, local_name: &LocalName) -> bool {
        if *local_name == local_name!("style") {
            return self.style_attribute_dependency;
        }

        self.attribute_dependencies
            .might_contain_hash(local_name.get_hash())
    }
    #[inline]
    fn normal_rules(&self, pseudo: Option<&PseudoElement>) -> Option<&SelectorMap<Rule>> {
        self.normal_rules.rules(pseudo)
    }

    #[inline]
    fn host_rules(&self, pseudo: Option<&PseudoElement>) -> Option<&SelectorMap<Rule>> {
        self.host_rules.as_ref().and_then(|d| d.rules(pseudo))
    }

    #[inline]
    fn slotted_rules(&self, pseudo: Option<&PseudoElement>) -> Option<&SelectorMap<Rule>> {
        self.slotted_rules.as_ref().and_then(|d| d.rules(pseudo))
    }

    /// Collects all the applicable media query results into `results`.
    ///
    /// This duplicates part of the logic in `add_stylesheet`, which is
    /// a bit unfortunate.
    ///
    /// FIXME(emilio): With a bit of smartness in
    /// `media_feature_affected_matches`, we could convert
    /// `EffectiveMediaQueryResults` into a vector without too much effort.
    fn collect_applicable_media_query_results_into<S>(
        device: &Device,
        stylesheet: &S,
        guard: &SharedRwLockReadGuard,
        results: &mut EffectiveMediaQueryResults,
    ) where
        S: StylesheetInDocument + ToMediaListKey + 'static,
    {
        if !stylesheet.enabled() || !stylesheet.is_effective_for_device(device, guard) {
            return;
        }

        debug!(" + {:?}", stylesheet);
        results.saw_effective(stylesheet);

        for rule in stylesheet.effective_rules(device, guard) {
            match *rule {
                CssRule::Import(ref lock) => {
                    let import_rule = lock.read_with(guard);
                    debug!(" + {:?}", import_rule.stylesheet.media(guard));
                    results.saw_effective(import_rule);
                },
                CssRule::Media(ref lock) => {
                    let media_rule = lock.read_with(guard);
                    debug!(" + {:?}", media_rule.media_queries.read_with(guard));
                    results.saw_effective(media_rule);
                },
                _ => {},
            }
        }
    }

    // Returns Err(..) to signify OOM
    fn add_stylesheet<S>(
        &mut self,
        device: &Device,
        quirks_mode: QuirksMode,
        stylesheet: &S,
        guard: &SharedRwLockReadGuard,
        rebuild_kind: SheetRebuildKind,
        mut precomputed_pseudo_element_decls: Option<&mut PrecomputedPseudoElementDeclarations>,
    ) -> Result<(), FailedAllocationError>
    where
        S: StylesheetInDocument + ToMediaListKey + 'static,
    {
        if !stylesheet.enabled() || !stylesheet.is_effective_for_device(device, guard) {
            return Ok(());
        }

        let origin = stylesheet.origin(guard);

        if rebuild_kind.should_rebuild_invalidation() {
            self.effective_media_query_results.saw_effective(stylesheet);
        }

        for rule in stylesheet.effective_rules(device, guard) {
            match *rule {
                CssRule::Style(ref locked) => {
                    let style_rule = locked.read_with(&guard);
                    self.num_declarations += style_rule.block.read_with(&guard).len();
                    for selector in &style_rule.selectors.0 {
                        self.num_selectors += 1;

                        let pseudo_element = selector.pseudo_element();

                        if let Some(pseudo) = pseudo_element {
                            if pseudo.is_precomputed() {
                                debug_assert!(selector.is_universal());
                                debug_assert!(matches!(origin, Origin::UserAgent));

                                precomputed_pseudo_element_decls
                                    .as_mut()
                                    .expect("Expected precomputed declarations for the UA level")
                                    .get_or_insert_with(&pseudo.canonical(), Vec::new)
                                    .push(ApplicableDeclarationBlock::new(
                                        StyleSource::from_rule(locked.clone()),
                                        self.rules_source_order,
                                        CascadeLevel::UANormal,
                                        selector.specificity(),
                                        0,
                                    ));
                                continue;
                            }
                        }

                        let hashes = AncestorHashes::new(&selector, quirks_mode);

                        let rule = Rule::new(
                            selector.clone(),
                            hashes,
                            locked.clone(),
                            self.rules_source_order,
                        );

                        if rebuild_kind.should_rebuild_invalidation() {
                            self.invalidation_map.note_selector(selector, quirks_mode)?;
                            let mut visitor = StylistSelectorVisitor {
                                needs_revalidation: false,
                                passed_rightmost_selector: false,
                                attribute_dependencies: &mut self.attribute_dependencies,
                                style_attribute_dependency: &mut self.style_attribute_dependency,
                                state_dependencies: &mut self.state_dependencies,
                                document_state_dependencies: &mut self.document_state_dependencies,
                                mapped_ids: &mut self.mapped_ids,
                            };

                            rule.selector.visit(&mut visitor);

                            if visitor.needs_revalidation {
                                self.selectors_for_cache_revalidation.insert(
                                    RevalidationSelectorAndHashes::new(
                                        rule.selector.clone(),
                                        rule.hashes.clone(),
                                    ),
                                    quirks_mode,
                                )?;
                            }
                        }

                        // NOTE(emilio): It's fine to look at :host and then at
                        // ::slotted(..), since :host::slotted(..) could never
                        // possibly match, as <slot> is not a valid shadow host.
                        let rules = if selector.is_featureless_host_selector_or_pseudo_element() {
                            self.host_rules
                                .get_or_insert_with(|| Box::new(Default::default()))
                        } else if selector.is_slotted() {
                            self.slotted_rules
                                .get_or_insert_with(|| Box::new(Default::default()))
                        } else {
                            &mut self.normal_rules
                        };

                        rules.insert(rule, pseudo_element, quirks_mode)?;
                    }
                    self.rules_source_order += 1;
                },
                CssRule::Import(ref lock) => {
                    if rebuild_kind.should_rebuild_invalidation() {
                        let import_rule = lock.read_with(guard);
                        self.effective_media_query_results
                            .saw_effective(import_rule);
                    }

                    // NOTE: effective_rules visits the inner stylesheet if
                    // appropriate.
                },
                CssRule::Media(ref lock) => {
                    if rebuild_kind.should_rebuild_invalidation() {
                        let media_rule = lock.read_with(guard);
                        self.effective_media_query_results.saw_effective(media_rule);
                    }
                },
                CssRule::Keyframes(ref keyframes_rule) => {
                    let keyframes_rule = keyframes_rule.read_with(guard);
                    debug!("Found valid keyframes rule: {:?}", *keyframes_rule);

                    // Don't let a prefixed keyframes animation override a non-prefixed one.
                    let needs_insertion = keyframes_rule.vendor_prefix.is_none() ||
                        self.animations
                            .get(keyframes_rule.name.as_atom())
                            .map_or(true, |rule| rule.vendor_prefix.is_some());
                    if needs_insertion {
                        let animation = KeyframesAnimation::from_keyframes(
                            &keyframes_rule.keyframes,
                            keyframes_rule.vendor_prefix.clone(),
                            guard,
                        );
                        debug!("Found valid keyframe animation: {:?}", animation);
                        self.animations
                            .try_insert(keyframes_rule.name.as_atom().clone(), animation)?;
                    }
                },
                #[cfg(feature = "gecko")]
                CssRule::FontFace(ref rule) => {
                    self.extra_data.add_font_face(rule);
                },
                #[cfg(feature = "gecko")]
                CssRule::FontFeatureValues(ref rule) => {
                    self.extra_data.add_font_feature_values(rule);
                },
                #[cfg(feature = "gecko")]
                CssRule::CounterStyle(ref rule) => {
                    self.extra_data.add_counter_style(guard, rule);
                },
                #[cfg(feature = "gecko")]
                CssRule::Page(ref rule) => {
                    self.extra_data.add_page(rule);
                },
                // We don't care about any other rule.
                _ => {},
            }
        }

        Ok(())
    }

    /// Returns whether all the media-feature affected values matched before and
    /// match now in the given stylesheet.
    pub fn media_feature_affected_matches<S>(
        &self,
        stylesheet: &S,
        guard: &SharedRwLockReadGuard,
        device: &Device,
        quirks_mode: QuirksMode,
    ) -> bool
    where
        S: StylesheetInDocument + ToMediaListKey + 'static,
    {
        use invalidation::media_queries::PotentiallyEffectiveMediaRules;

        let effective_now = stylesheet.is_effective_for_device(device, guard);

        let effective_then = self.effective_media_query_results.was_effective(stylesheet);

        if effective_now != effective_then {
            debug!(
                " > Stylesheet {:?} changed -> {}, {}",
                stylesheet.media(guard),
                effective_then,
                effective_now
            );
            return false;
        }

        if !effective_now {
            return true;
        }

        let mut iter = stylesheet.iter_rules::<PotentiallyEffectiveMediaRules>(device, guard);

        while let Some(rule) = iter.next() {
            match *rule {
                CssRule::Style(..) |
                CssRule::Namespace(..) |
                CssRule::FontFace(..) |
                CssRule::CounterStyle(..) |
                CssRule::Supports(..) |
                CssRule::Keyframes(..) |
                CssRule::Page(..) |
                CssRule::Viewport(..) |
                CssRule::Document(..) |
                CssRule::FontFeatureValues(..) => {
                    // Not affected by device changes.
                    continue;
                },
                CssRule::Import(ref lock) => {
                    let import_rule = lock.read_with(guard);
                    let effective_now = import_rule
                        .stylesheet
                        .is_effective_for_device(&device, guard);
                    let effective_then = self.effective_media_query_results
                        .was_effective(import_rule);
                    if effective_now != effective_then {
                        debug!(
                            " > @import rule {:?} changed {} -> {}",
                            import_rule.stylesheet.media(guard),
                            effective_then,
                            effective_now
                        );
                        return false;
                    }

                    if !effective_now {
                        iter.skip_children();
                    }
                },
                CssRule::Media(ref lock) => {
                    let media_rule = lock.read_with(guard);
                    let mq = media_rule.media_queries.read_with(guard);
                    let effective_now = mq.evaluate(device, quirks_mode);
                    let effective_then =
                        self.effective_media_query_results.was_effective(media_rule);

                    if effective_now != effective_then {
                        debug!(
                            " > @media rule {:?} changed {} -> {}",
                            mq,
                            effective_then,
                            effective_now
                        );
                        return false;
                    }

                    if !effective_now {
                        iter.skip_children();
                    }
                },
            }
        }

        true
    }

    /// Clears the cascade data, but not the invalidation data.
    fn clear_cascade_data(&mut self) {
        self.normal_rules.clear();
        if let Some(ref mut slotted_rules) = self.slotted_rules {
            slotted_rules.clear();
        }
        self.animations.clear();
        self.extra_data.clear();
        self.rules_source_order = 0;
        self.num_selectors = 0;
        self.num_declarations = 0;
    }

    fn clear(&mut self) {
        self.clear_cascade_data();
        self.invalidation_map.clear();
        self.attribute_dependencies.clear();
        self.style_attribute_dependency = false;
        self.state_dependencies = ElementState::empty();
        self.document_state_dependencies = DocumentState::empty();
        self.mapped_ids.clear();
        self.selectors_for_cache_revalidation.clear();
        self.effective_media_query_results.clear();
    }

    /// Measures heap usage.
    #[cfg(feature = "gecko")]
    fn add_size_of(&self, ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
        self.normal_rules.add_size_of(ops, sizes);
        if let Some(ref slotted_rules) = self.slotted_rules {
            slotted_rules.add_size_of(ops, sizes);
        }
        sizes.mInvalidationMap += self.invalidation_map.size_of(ops);
        sizes.mRevalidationSelectors += self.selectors_for_cache_revalidation.size_of(ops);
        sizes.mOther += self.animations.size_of(ops);
        sizes.mOther += self.effective_media_query_results.size_of(ops);
        sizes.mOther += self.extra_data.size_of(ops);
    }
}

impl Default for CascadeData {
    fn default() -> Self {
        CascadeData::new()
    }
}

/// A rule, that wraps a style rule, but represents a single selector of the
/// rule.
#[derive(Clone, Debug, MallocSizeOf)]
pub struct Rule {
    /// The selector this struct represents. We store this and the
    /// any_{important,normal} booleans inline in the Rule to avoid
    /// pointer-chasing when gathering applicable declarations, which
    /// can ruin performance when there are a lot of rules.
    #[ignore_malloc_size_of = "CssRules have primary refs, we measure there"]
    pub selector: Selector<SelectorImpl>,

    /// The ancestor hashes associated with the selector.
    pub hashes: AncestorHashes,

    /// The source order this style rule appears in. Note that we only use
    /// three bytes to store this value in ApplicableDeclarationsBlock, so
    /// we could repurpose that storage here if we needed to.
    pub source_order: u32,

    /// The actual style rule.
    #[cfg_attr(feature = "gecko",
               ignore_malloc_size_of = "Secondary ref. Primary ref is in StyleRule under Stylesheet.")]
    #[cfg_attr(feature = "servo", ignore_malloc_size_of = "Arc")]
    pub style_rule: Arc<Locked<StyleRule>>,
}

impl SelectorMapEntry for Rule {
    fn selector(&self) -> SelectorIter<SelectorImpl> {
        self.selector.iter()
    }
}

impl Rule {
    /// Returns the specificity of the rule.
    pub fn specificity(&self) -> u32 {
        self.selector.specificity()
    }

    /// Turns this rule into an `ApplicableDeclarationBlock` for the given
    /// cascade level.
    pub fn to_applicable_declaration_block(
        &self,
        level: CascadeLevel,
        shadow_cascade_order: ShadowCascadeOrder,
    ) -> ApplicableDeclarationBlock {
        let source = StyleSource::from_rule(self.style_rule.clone());
        ApplicableDeclarationBlock::new(source, self.source_order, level, self.specificity(), shadow_cascade_order)
    }

    /// Creates a new Rule.
    pub fn new(
        selector: Selector<SelectorImpl>,
        hashes: AncestorHashes,
        style_rule: Arc<Locked<StyleRule>>,
        source_order: u32,
    ) -> Self {
        Rule {
            selector: selector,
            hashes: hashes,
            style_rule: style_rule,
            source_order: source_order,
        }
    }
}

/// A function to be able to test the revalidation stuff.
pub fn needs_revalidation_for_testing(s: &Selector<SelectorImpl>) -> bool {
    let mut attribute_dependencies = NonCountingBloomFilter::new();
    let mut mapped_ids = NonCountingBloomFilter::new();
    let mut style_attribute_dependency = false;
    let mut state_dependencies = ElementState::empty();
    let mut document_state_dependencies = DocumentState::empty();
    let mut visitor = StylistSelectorVisitor {
        needs_revalidation: false,
        passed_rightmost_selector: false,
        attribute_dependencies: &mut attribute_dependencies,
        style_attribute_dependency: &mut style_attribute_dependency,
        state_dependencies: &mut state_dependencies,
        document_state_dependencies: &mut document_state_dependencies,
        mapped_ids: &mut mapped_ids,
    };
    s.visit(&mut visitor);
    visitor.needs_revalidation
}