diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 8eee0a43be29..af2b0ab6ff51 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -16,6 +16,7 @@
 #include "mozilla/DebugOnly.h"
 #include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/StaticPrefs_layout.h"
+#include "mozilla/StaticPrefs_full_screen_api.h"
 #include "mozilla/dom/Animation.h"
 #include "mozilla/dom/Attr.h"
 #include "mozilla/dom/BindContext.h"
@@ -2517,6 +2518,12 @@ bool Element::ParseAttribute(int32_t aNamespaceID, nsAtom* aAttribute,
       return true;
     }
 
+    if (aAttribute == nsGkAtoms::exportparts &&
+        StaticPrefs::layout_css_shadow_parts_enabled()) {
+      aResult.ParsePartMapping(aValue);
+      return true;
+    }
+
     if (aAttribute == nsGkAtoms::id) {
       // Store id as an atom.  id="" means that the element has no id,
       // not that it has an emptystring as the id.
diff --git a/dom/base/nsAttrValue.cpp b/dom/base/nsAttrValue.cpp
index c000eebb80b8..8875d23680b7 100644
--- a/dom/base/nsAttrValue.cpp
+++ b/dom/base/nsAttrValue.cpp
@@ -20,6 +20,7 @@
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/ServoBindingTypes.h"
 #include "mozilla/ServoUtils.h"
+#include "mozilla/ShadowParts.h"
 #include "mozilla/DeclarationBlock.h"
 #include "nsContentUtils.h"
 #include "nsReadableUtils.h"
@@ -281,6 +282,7 @@ void nsAttrValue::SetTo(const nsAttrValue& aOther) {
       cont->mValue.mColor = otherCont->mValue.mColor;
       break;
     }
+    case eShadowParts:
     case eCSSDeclaration: {
       MOZ_CRASH("These should be refcounted!");
     }
@@ -301,9 +303,10 @@ void nsAttrValue::SetTo(const nsAttrValue& aOther) {
       break;
     }
     case eIntMarginValue: {
-      if (otherCont->mValue.mIntMargin)
+      if (otherCont->mValue.mIntMargin) {
         cont->mValue.mIntMargin =
             new nsIntMargin(*otherCont->mValue.mIntMargin);
+      }
       break;
     }
     default: {
@@ -1155,7 +1158,7 @@ void nsAttrValue::ParseAtomArray(const nsAString& aValue) {
 void nsAttrValue::ParseStringOrAtom(const nsAString& aValue) {
   uint32_t len = aValue.Length();
   // Don't bother with atoms if it's an empty string since
-  // we can store those efficently anyway.
+  // we can store those efficiently anyway.
   if (len && len <= NS_ATTRVALUE_MAX_STRINGLENGTH_ATOM) {
     ParseAtom(aValue);
   } else {
@@ -1163,6 +1166,17 @@ void nsAttrValue::ParseStringOrAtom(const nsAString& aValue) {
   }
 }
 
+void nsAttrValue::ParsePartMapping(const nsAString& aValue) {
+  ResetIfSet();
+  MiscContainer* cont = EnsureEmptyMiscContainer();
+
+  cont->mType = eShadowParts;
+  cont->mValue.mShadowParts = new ShadowParts(ShadowParts::Parse(aValue));
+  NS_ADDREF(cont);
+  SetMiscAtomOrString(&aValue);
+  MOZ_ASSERT(cont->mValue.mRefCount == 1);
+}
+
 void nsAttrValue::SetIntValueAndType(int32_t aValue, ValueType aType,
                                      const nsAString* aStringValue) {
   if (aStringValue || aValue > NS_ATTRVALUE_INTEGERTYPE_MAXVALUE ||
@@ -1740,6 +1754,12 @@ MiscContainer* nsAttrValue::ClearMiscContainer() {
           NS_RELEASE(cont->mValue.mCSSDeclaration);
           break;
         }
+        case eShadowParts: {
+          MOZ_ASSERT(cont->mValue.mRefCount == 1);
+          cont->Release();
+          delete cont->mValue.mShadowParts;
+          break;
+        }
         case eURL: {
           NS_RELEASE(cont->mValue.mURL);
           break;
diff --git a/dom/base/nsAttrValue.h b/dom/base/nsAttrValue.h
index 0dd4c227e9fc..29d2a0cbd3cd 100644
--- a/dom/base/nsAttrValue.h
+++ b/dom/base/nsAttrValue.h
@@ -101,6 +101,10 @@ class nsAttrValue {
     eAtomArray,
     eDoubleValue,
     eIntMarginValue,
+    // eShadowParts is refcounted in the misc container, as we do copy attribute
+    // values quite a bit (for example to process style invalidation), and the
+    // underlying value could get expensive to copy.
+    eShadowParts,
     eSVGIntegerPair,
     eSVGTypesBegin = eSVGIntegerPair,
     eSVGOrient,
@@ -256,6 +260,13 @@ class nsAttrValue {
   void ParseAtomArray(const nsAString& aValue);
   void ParseStringOrAtom(const nsAString& aValue);
 
+  /**
+   * Parses an exportparts attribute.
+   *
+   * https://drafts.csswg.org/css-shadow-parts/#parsing-mapping-list
+   */
+  void ParsePartMapping(const nsAString&);
+
   /**
    * Structure for a mapping from int (enum) values to strings.  When you use
    * it you generally create an array of them.
diff --git a/dom/base/nsAttrValueInlines.h b/dom/base/nsAttrValueInlines.h
index 274b13fb7604..5d74d241e9cd 100644
--- a/dom/base/nsAttrValueInlines.h
+++ b/dom/base/nsAttrValueInlines.h
@@ -14,6 +14,10 @@
 #include "mozilla/Attributes.h"
 #include "mozilla/ServoUtils.h"
 
+namespace mozilla {
+class ShadowParts;
+}
+
 struct MiscContainer final {
   typedef nsAttrValue::ValueType ValueType;
 
@@ -41,6 +45,7 @@ struct MiscContainer final {
         nsIURI* mURL;
         mozilla::AtomArray* mAtomArray;
         nsIntMargin* mIntMargin;
+        const mozilla::ShadowParts* mShadowParts;
         const mozilla::SVGAnimatedIntegerPair* mSVGAnimatedIntegerPair;
         const mozilla::SVGAnimatedLength* mSVGLength;
         const mozilla::SVGAnimatedNumberPair* mSVGAnimatedNumberPair;
@@ -95,7 +100,8 @@ struct MiscContainer final {
     // Nothing stops us from refcounting (and sharing) other types of
     // MiscContainer (except eDoubleValue types) but there's no compelling
     // reason to.
-    return mType == nsAttrValue::eCSSDeclaration;
+    return mType == nsAttrValue::eCSSDeclaration ||
+           mType == nsAttrValue::eShadowParts;
   }
 
   inline int32_t AddRef() {
diff --git a/layout/style/ShadowParts.cpp b/layout/style/ShadowParts.cpp
new file mode 100644
index 000000000000..16fc5c8ec231
--- /dev/null
+++ b/layout/style/ShadowParts.cpp
@@ -0,0 +1,114 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#include "ShadowParts.h"
+#include "nsContentUtils.h"
+#include "nsString.h"
+
+namespace mozilla {
+
+static bool IsSpace(char16_t aChar) {
+  return nsContentUtils::IsHTMLWhitespace(aChar);
+};
+
+using SingleMapping = std::pair<RefPtr<nsAtom>, RefPtr<nsAtom>>;
+
+// https://drafts.csswg.org/css-shadow-parts/#parsing-mapping
+//
+// Returns null on both tokens to signal an error.
+static SingleMapping ParseSingleMapping(const nsAString& aString) {
+  const char16_t* c = aString.BeginReading();
+  const char16_t* end = aString.EndReading();
+
+  const auto CollectASequenceOfSpaces = [&c, end]() {
+    while (c != end && IsSpace(*c)) {
+      ++c;
+    }
+  };
+
+  const auto CollectToken = [&c, end]() -> RefPtr<nsAtom> {
+    const char16_t* t = c;
+    while (c != end && !IsSpace(*c) && *c != ':') {
+      ++c;
+    }
+    if (c == t) {
+      return nullptr;
+    }
+    return NS_AtomizeMainThread(Substring(t, c));
+  };
+
+  // Steps 1 and 2 are variable declarations.
+  //
+  // 3. Collect a sequence of code points that are space characters.
+  CollectASequenceOfSpaces();
+
+  // 4. Collect a sequence of code points that are not space characters or
+  // U+003A COLON characters, and call the result first token.
+  RefPtr<nsAtom> firstToken = CollectToken();
+
+  // 5. If first token is empty then return error.
+  if (!firstToken) {
+    return {nullptr, nullptr};
+  }
+
+  // 6. Collect a sequence of code points that are space characters.
+  CollectASequenceOfSpaces();
+
+  // 7. If the end of the input has been reached, return the pair first
+  // token/first token.
+  if (c == end) {
+    return {firstToken, firstToken};
+  }
+
+  // 8. If character at position is not a U+003A COLON character, return error.
+  if (*c != ':') {
+    return {nullptr, nullptr};
+  }
+
+  // 9. Consume the U+003A COLON character.
+  ++c;
+
+  // 10. Collect a sequence of code points that are space characters.
+  CollectASequenceOfSpaces();
+
+  // 11. Collect a sequence of code points that are not space characters or
+  // U+003A COLON characters. and let second token be the result.
+  RefPtr<nsAtom> secondToken = CollectToken();
+
+  // 12. If second token is empty then return error.
+  if (!secondToken) {
+    return {nullptr, nullptr};
+  }
+
+  // 13. Collect a sequence of code points that are space characters.
+  CollectASequenceOfSpaces();
+
+  // 14. If position is not past the end of input then return error.
+  if (c != end) {
+    return {nullptr, nullptr};
+  }
+
+  // 15. Return the pair first token/second token.
+  return {std::move(firstToken), std::move(secondToken)};
+}
+
+// https://drafts.csswg.org/css-shadow-parts/#parsing-mapping-list
+ShadowParts ShadowParts::Parse(const nsAString& aString) {
+  ShadowParts parts;
+
+  for (const auto& substring : aString.Split(',')) {
+    auto mapping = ParseSingleMapping(substring);
+    if (!mapping.first) {
+      MOZ_ASSERT(!mapping.second);
+      continue;
+    }
+    parts.mMappings.GetOrInsert(mapping.first) = std::move(mapping.second);
+  }
+
+  return parts;
+}
+
+}  // namespace mozilla
diff --git a/layout/style/ShadowParts.h b/layout/style/ShadowParts.h
new file mode 100644
index 000000000000..7be97386cf50
--- /dev/null
+++ b/layout/style/ShadowParts.h
@@ -0,0 +1,32 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef mozilla_ShadowParts_h
+#define mozilla_ShadowParts_h
+
+#include "nsAtom.h"
+#include "nsTHashtable.h"
+#include "nsRefPtrHashtable.h"
+#include "nsStringFwd.h"
+
+namespace mozilla {
+
+class ShadowParts final {
+ public:
+  ShadowParts(ShadowParts&&) = default;
+  ShadowParts(const ShadowParts&) = delete;
+
+  static ShadowParts Parse(const nsAString&);
+
+ private:
+  ShadowParts() = default;
+
+  nsRefPtrHashtable<nsRefPtrHashKey<nsAtom>, nsAtom> mMappings;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_ShadowParts_h
diff --git a/layout/style/moz.build b/layout/style/moz.build
index 3d3d18eb731c..3f3dc5477106 100644
--- a/layout/style/moz.build
+++ b/layout/style/moz.build
@@ -107,6 +107,7 @@ EXPORTS.mozilla += [
     'ServoTraversalStatistics.h',
     'ServoTypes.h',
     'ServoUtils.h',
+    'ShadowParts.h',
     'StyleAnimationValue.h',
     'StyleColorInlines.h',
     'StyleSheet.h',
@@ -221,6 +222,7 @@ UNIFIED_SOURCES += [
     'ServoCSSRuleList.cpp',
     'ServoElementSnapshot.cpp',
     'ServoStyleSet.cpp',
+    'ShadowParts.cpp',
     'StreamLoader.cpp',
     'StyleAnimationValue.cpp',
     'StyleColor.cpp',
diff --git a/layout/style/nsLayoutStylesheetCache.cpp b/layout/style/nsLayoutStylesheetCache.cpp
index 4f9e09deb36c..3300baa0df38 100644
--- a/layout/style/nsLayoutStylesheetCache.cpp
+++ b/layout/style/nsLayoutStylesheetCache.cpp
@@ -424,7 +424,7 @@ void nsLayoutStylesheetCache::InitSharedSheetsInParent() {
   // TODO(heycam): This won't be true on Windows unless we allow creating the
   // shared memory with SEC_RESERVE so that the pages are reserved but not
   // committed.
-  size_t pageSize = ipc::SharedMemory::SystemPageSize();
+  size_t pageSize = mozilla::ipc::SharedMemory::SystemPageSize();
   mUsedSharedMemory =
       (Servo_SharedMemoryBuilder_GetLength(builder.get()) + pageSize - 1) &
       ~(pageSize - 1);
diff --git a/xpcom/ds/StaticAtoms.py b/xpcom/ds/StaticAtoms.py
index c794c945f617..ac14cbd6a7f5 100644
--- a/xpcom/ds/StaticAtoms.py
+++ b/xpcom/ds/StaticAtoms.py
@@ -413,6 +413,7 @@ STATIC_ATOMS = [
     Atom("event", "event"),
     Atom("events", "events"),
     Atom("excludeResultPrefixes", "exclude-result-prefixes"),
+    Atom("exportparts", "exportparts"),
     Atom("extends", "extends"),
     Atom("extensionElementPrefixes", "extension-element-prefixes"),
     Atom("face", "face"),