diff --git a/devtools/server/actors/animation-type-longhand.js b/devtools/server/actors/animation-type-longhand.js
index 043d1243443b..5966179331b8 100644
--- a/devtools/server/actors/animation-type-longhand.js
+++ b/devtools/server/actors/animation-type-longhand.js
@@ -87,6 +87,7 @@ exports.ANIMATION_TYPE_FOR_LONGHANDS = [
       "image-orientation",
       "image-rendering",
       "ime-mode",
+      "-moz-inert",
       "initial-letter",
       "isolation",
       "justify-content",
diff --git a/dom/events/EventStates.h b/dom/events/EventStates.h
index 59de4c556959..3aa7a055ae7c 100644
--- a/dom/events/EventStates.h
+++ b/dom/events/EventStates.h
@@ -294,6 +294,8 @@ class EventStates {
 #define NS_EVENT_STATE_FOCUS_VISIBLE NS_DEFINE_EVENT_STATE_MACRO(52)
 // Modal <dialog> element
 #define NS_EVENT_STATE_MODAL_DIALOG NS_DEFINE_EVENT_STATE_MACRO(53)
+// Inert subtrees
+#define NS_EVENT_STATE_MOZINERT NS_DEFINE_EVENT_STATE_MACRO(54)
 
 /**
  * NOTE: do not go over 63 without updating EventStates::InternalType!
@@ -329,7 +331,8 @@ class EventStates {
    NS_EVENT_STATE_DRAGOVER | NS_EVENT_STATE_FOCUS | NS_EVENT_STATE_FOCUSRING | \
    NS_EVENT_STATE_FOCUS_WITHIN | NS_EVENT_STATE_FULLSCREEN |                   \
    NS_EVENT_STATE_HOVER | NS_EVENT_STATE_URLTARGET |                           \
-   NS_EVENT_STATE_FOCUS_VISIBLE | NS_EVENT_STATE_MODAL_DIALOG)
+   NS_EVENT_STATE_FOCUS_VISIBLE | NS_EVENT_STATE_MODAL_DIALOG |                \
+   NS_EVENT_STATE_MOZINERT)
 
 #define INTRINSIC_STATES (~EXTERNALLY_MANAGED_STATES)
 
diff --git a/layout/generic/nsIFrame.cpp b/layout/generic/nsIFrame.cpp
index 31445731f321..a4380b57f79f 100644
--- a/layout/generic/nsIFrame.cpp
+++ b/layout/generic/nsIFrame.cpp
@@ -9761,7 +9761,8 @@ bool nsIFrame::IsFocusable(int32_t* aTabIndex, bool aWithMouse) {
 
   if (mContent && mContent->IsElement() && IsVisibleConsideringAncestors() &&
       Style()->GetPseudoType() != PseudoStyleType::anonymousFlexItem &&
-      Style()->GetPseudoType() != PseudoStyleType::anonymousGridItem) {
+      Style()->GetPseudoType() != PseudoStyleType::anonymousGridItem &&
+      StyleUI()->mInert != StyleInert::Inert) {
     const nsStyleUI* ui = StyleUI();
     if (ui->mUserFocus != StyleUserFocus::Ignore &&
         ui->mUserFocus != StyleUserFocus::None) {
diff --git a/layout/style/ServoBindings.toml b/layout/style/ServoBindings.toml
index 369d62ae1a30..46202d1b81cd 100644
--- a/layout/style/ServoBindings.toml
+++ b/layout/style/ServoBindings.toml
@@ -97,6 +97,7 @@ rusty-enums = [
     "mozilla::StyleFloat",
     "mozilla::StyleImageOrientation",
     "mozilla::StyleImageRendering",
+    "mozilla::StyleInert",
     "mozilla::StyleUserModify",
     "mozilla::StyleUserInput",
     "mozilla::StyleBoxDirection",
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index 278fff536f57..270d0727851c 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -286,6 +286,12 @@ enum class StyleUserModify : uint8_t {
   WriteOnly,
 };
 
+// -moz-inert
+enum class StyleInert : uint8_t {
+  None,
+  Inert,
+};
+
 // -moz-window-dragging
 enum class StyleWindowDragging : uint8_t {
   Default,
diff --git a/layout/style/nsStyleStruct.cpp b/layout/style/nsStyleStruct.cpp
index 175277165a69..a07732e65c8f 100644
--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -3107,7 +3107,8 @@ LogicalSide nsStyleText::TextEmphasisSide(WritingMode aWM) const {
 //
 
 nsStyleUI::nsStyleUI(const Document& aDocument)
-    : mUserInput(StyleUserInput::Auto),
+    : mInert(StyleInert::None),
+      mUserInput(StyleUserInput::Auto),
       mUserModify(StyleUserModify::ReadOnly),
       mUserFocus(StyleUserFocus::None),
       mPointerEvents(StylePointerEvents::Auto),
@@ -3118,7 +3119,8 @@ nsStyleUI::nsStyleUI(const Document& aDocument)
 }
 
 nsStyleUI::nsStyleUI(const nsStyleUI& aSource)
-    : mUserInput(aSource.mUserInput),
+    : mInert(aSource.mInert),
+      mUserInput(aSource.mUserInput),
       mUserModify(aSource.mUserModify),
       mUserFocus(aSource.mUserFocus),
       mPointerEvents(aSource.mPointerEvents),
@@ -3179,7 +3181,7 @@ nsChangeHint nsStyleUI::CalcDifference(const nsStyleUI& aNewData) const {
     }
   }
 
-  if (mUserFocus != aNewData.mUserFocus) {
+  if (mUserFocus != aNewData.mUserFocus || mInert != aNewData.mInert) {
     hint |= nsChangeHint_NeutralChange;
   }
 
diff --git a/layout/style/nsStyleStruct.h b/layout/style/nsStyleStruct.h
index 69f1cac2c65e..3998ca66df11 100644
--- a/layout/style/nsStyleStruct.h
+++ b/layout/style/nsStyleStruct.h
@@ -1730,6 +1730,7 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleUI {
 
   nsChangeHint CalcDifference(const nsStyleUI& aNewData) const;
 
+  mozilla::StyleInert mInert;
   mozilla::StyleUserInput mUserInput;
   mozilla::StyleUserModify mUserModify;  // (modify-content)
   mozilla::StyleUserFocus mUserFocus;    // (auto-select)
diff --git a/layout/style/res/ua.css b/layout/style/res/ua.css
index 1a20b3d8b562..a63ca0b964e5 100644
--- a/layout/style/res/ua.css
+++ b/layout/style/res/ua.css
@@ -153,6 +153,11 @@
   outline: 1px dotted;
 }
 
+/* Inert subtrees */
+*|*:-moz-inert {
+  -moz-inert: inert;
+}
+
 /* Miscellaneous */
 
 *|*::-moz-cell-content {
diff --git a/layout/style/test/ListCSSProperties.cpp b/layout/style/test/ListCSSProperties.cpp
index e5ee5e737244..a9d45eba041b 100644
--- a/layout/style/test/ListCSSProperties.cpp
+++ b/layout/style/test/ListCSSProperties.cpp
@@ -94,6 +94,7 @@ const char* gInaccessibleProperties[] = {
     "-moz-context-properties",
     "-moz-control-character-visibility",
     "-moz-default-appearance",
+    "-moz-inert",
     "-moz-list-reversed",  // parsed by UA sheets only
     "-moz-script-level",   // parsed by UA sheets only
     "-moz-script-size-multiplier",
diff --git a/layout/style/test/test_non_content_accessible_properties.html b/layout/style/test/test_non_content_accessible_properties.html
index 4e1e2a6dce0a..7a6e0f5d4ff5 100644
--- a/layout/style/test/test_non_content_accessible_properties.html
+++ b/layout/style/test/test_non_content_accessible_properties.html
@@ -23,6 +23,7 @@ const NON_CONTENT_ACCESSIBLE_PROPERTIES = [
   "-moz-min-font-size-ratio",
   "-moz-script-size-multiplier",
   "-moz-default-appearance",
+  "-moz-inert",
   // TODO(emilio): Whenever we stop using `-moz-binding` in a gazillion tests
   // we should add it here.
 ];
diff --git a/layout/style/test/test_selectors.html b/layout/style/test/test_selectors.html
index 72486caeaee4..380756b63d38 100644
--- a/layout/style/test/test_selectors.html
+++ b/layout/style/test/test_selectors.html
@@ -1137,6 +1137,7 @@ function runTests() {
     test_unbalanced_unparseable(":-moz-handler-crashed");
 
     // We're not in a UA sheet, so this should be invalid.
+    test_balanced_unparseable(":-moz-inert");
     test_balanced_unparseable(":-moz-native-anonymous");
     test_balanced_unparseable(":-moz-table-border-nonzero");
 
diff --git a/servo/components/style/element_state.rs b/servo/components/style/element_state.rs
index 027ebc839b4f..da44186b44ba 100644
--- a/servo/components/style/element_state.rs
+++ b/servo/components/style/element_state.rs
@@ -145,6 +145,9 @@ bitflags! {
         ///
         /// https://html.spec.whatwg.org/multipage/#centered-alignment
         const IN_MODAL_DIALOG_STATE = 1 << 53;
+
+        /// https://html.spec.whatwg.org/multipage/interaction.html#inert-subtrees
+        const IN_MOZINERT_STATE = 1 << 54;
     }
 }
 
diff --git a/servo/components/style/gecko/non_ts_pseudo_class_list.rs b/servo/components/style/gecko/non_ts_pseudo_class_list.rs
index 034ce0694ad7..bdd994c70955 100644
--- a/servo/components/style/gecko/non_ts_pseudo_class_list.rs
+++ b/servo/components/style/gecko/non_ts_pseudo_class_list.rs
@@ -48,6 +48,7 @@ macro_rules! apply_non_ts_list {
                 ("-moz-drag-over", MozDragOver, IN_DRAGOVER_STATE, _),
                 ("target", Target, IN_TARGET_STATE, _),
                 ("indeterminate", Indeterminate, IN_INDETERMINATE_STATE, _),
+                ("-moz-inert", MozInert, IN_MOZINERT_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 ("-moz-devtools-highlighted", MozDevtoolsHighlighted, IN_DEVTOOLS_HIGHLIGHTED_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 ("-moz-styleeditor-transitioning", MozStyleeditorTransitioning, IN_STYLEEDITOR_TRANSITIONING_STATE, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
                 ("fullscreen", Fullscreen, IN_FULLSCREEN_STATE, _),
diff --git a/servo/components/style/gecko/wrapper.rs b/servo/components/style/gecko/wrapper.rs
index 0b2547edc46e..218642d304f9 100644
--- a/servo/components/style/gecko/wrapper.rs
+++ b/servo/components/style/gecko/wrapper.rs
@@ -2016,6 +2016,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::Checked |
             NonTSPseudoClass::Fullscreen |
             NonTSPseudoClass::Indeterminate |
+            NonTSPseudoClass::MozInert |
             NonTSPseudoClass::PlaceholderShown |
             NonTSPseudoClass::Target |
             NonTSPseudoClass::Valid |
diff --git a/servo/components/style/properties/longhands/inherited_ui.mako.rs b/servo/components/style/properties/longhands/inherited_ui.mako.rs
index bbe342096471..cb48acaa1fe9 100644
--- a/servo/components/style/properties/longhands/inherited_ui.mako.rs
+++ b/servo/components/style/properties/longhands/inherited_ui.mako.rs
@@ -29,6 +29,17 @@ ${helpers.single_keyword(
     gecko_enum_prefix="StylePointerEvents",
 )}
 
+${helpers.single_keyword(
+    "-moz-inert",
+    "none inert",
+    engines="gecko",
+    gecko_ffi_name="mInert",
+    gecko_enum_prefix="StyleInert",
+    animation_value_type="discrete",
+    enabled_in="ua",
+    spec="Nonstandard (https://html.spec.whatwg.org/multipage/interaction.html#inert-subtrees)",
+)}
+
 ${helpers.single_keyword(
     "-moz-user-input",
     "auto none",
diff --git a/servo/components/style/servo/selector_parser.rs b/servo/components/style/servo/selector_parser.rs
index a3a3ca7c2b86..3652eef0058a 100644
--- a/servo/components/style/servo/selector_parser.rs
+++ b/servo/components/style/servo/selector_parser.rs
@@ -347,6 +347,7 @@ impl ToCss for NonTSPseudoClass {
             Fullscreen => ":fullscreen",
             Hover => ":hover",
             Indeterminate => ":indeterminate",
+            MozInert => ":-moz-inert",
             Link => ":link",
             PlaceholderShown => ":placeholder-shown",
             ReadWrite => ":read-write",
@@ -436,6 +437,7 @@ impl<'a, 'i> ::selectors::Parser<'i> for SelectorParser<'a> {
             "fullscreen" => Fullscreen,
             "hover" => Hover,
             "indeterminate" => Indeterminate,
+            "-moz-inert" => MozInert,
             "link" => Link,
             "placeholder-shown" => PlaceholderShown,
             "read-write" => ReadWrite,
diff --git a/servo/components/style/style_adjuster.rs b/servo/components/style/style_adjuster.rs
index 8ab4aea3d216..fb5fae96bd0d 100644
--- a/servo/components/style/style_adjuster.rs
+++ b/servo/components/style/style_adjuster.rs
@@ -153,6 +153,56 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
         }
     }
 
+    /// https://html.spec.whatwg.org/multipage/interaction.html#inert-subtrees
+    ///
+    ///    If -moz-inert is applied then add:
+    ///        -moz-user-focus: none;
+    ///        -moz-user-input: none;
+    ///        -moz-user-modify: read-only;
+    ///        user-select: none;
+    ///        pointer-events: none;
+    ///        cursor: default;
+    fn adjust_for_inert(&mut self) {
+        use properties::longhands::_moz_inert::computed_value::T as Inert;
+        use properties::longhands::_moz_user_focus::computed_value::T as UserFocus;
+        use properties::longhands::_moz_user_input::computed_value::T as UserInput;
+        use properties::longhands::_moz_user_modify::computed_value::T as UserModify;
+        use properties::longhands::pointer_events::computed_value::T as PointerEvents;
+        use properties::longhands::cursor::computed_value::T as Cursor;
+        use crate::values::specified::ui::CursorKind;
+        use crate::values::specified::ui::UserSelect;
+
+        let needs_update = {
+            let ui = self.style.get_inherited_ui();
+            if ui.clone__moz_inert() == Inert::None {
+                return;
+            }
+
+            ui.clone__moz_user_focus() != UserFocus::None ||
+                ui.clone__moz_user_input() != UserInput::None ||
+                ui.clone__moz_user_modify() != UserModify::ReadOnly ||
+                ui.clone_pointer_events() != PointerEvents::None ||
+                ui.clone_cursor().keyword != CursorKind::Default ||
+                ui.clone_cursor().images != Default::default()
+        };
+
+        if needs_update {
+            let ui = self.style.mutate_inherited_ui();
+            ui.set__moz_user_focus(UserFocus::None);
+            ui.set__moz_user_input(UserInput::None);
+            ui.set__moz_user_modify(UserModify::ReadOnly);
+            ui.set_pointer_events(PointerEvents::None);
+            ui.set_cursor(Cursor {
+                images: Default::default(),
+                keyword: CursorKind::Default,
+            });
+        }
+
+        if self.style.get_ui().clone_user_select() != UserSelect::None {
+            self.style.mutate_ui().set_user_select(UserSelect::None);
+        }
+    }
+
     /// Whether we should skip any item-based display property blockification on
     /// this element.
     fn skip_item_display_fixup<E>(&self, element: Option<E>) -> bool
@@ -855,6 +905,7 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
         #[cfg(feature = "gecko")]
         {
             self.adjust_for_appearance(element);
+            self.adjust_for_inert();
         }
         self.set_bits();
     }