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diff --git a/layout/style/nsCSSPseudoElements.h b/layout/style/nsCSSPseudoElements.h
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+/* -*- 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/. */
+
+/* atom list for CSS pseudo-elements */
+
+#ifndef nsCSSPseudoElements_h___
+#define nsCSSPseudoElements_h___
+
+#include "nsGkAtoms.h"
+#include "mozilla/CSSEnabledState.h"
+#include "mozilla/PseudoStyleType.h"
+#include "mozilla/StaticPrefs_dom.h"
+#include "mozilla/StaticPrefs_layout.h"
+
+// Is this pseudo-element a CSS2 pseudo-element that can be specified
+// with the single colon syntax (in addition to the double-colon syntax,
+// which can be used for all pseudo-elements)?
+//
+// Note: We also rely on this for IsEagerlyCascadedInServo.
+#define CSS_PSEUDO_ELEMENT_IS_CSS2 (1 << 0)
+// Is this pseudo-element a pseudo-element that can contain other
+// elements?
+// (Currently pseudo-elements are either leaves of the tree (relative to
+// real elements) or they contain other elements in a non-tree-like
+// manner (i.e., like incorrectly-nested start and end tags).  It's
+// possible that in the future there might be container pseudo-elements
+// that form a properly nested tree structure.  If that happens, we
+// should probably split this flag into two.)
+#define CSS_PSEUDO_ELEMENT_CONTAINS_ELEMENTS (1 << 1)
+// Flag to add the ability to take into account style attribute set for the
+// pseudo element (by default it's ignored).
+#define CSS_PSEUDO_ELEMENT_SUPPORTS_STYLE_ATTRIBUTE (1 << 2)
+// Flag that indicate the pseudo-element supports a user action pseudo-class
+// following it, such as :active or :hover.  This would normally correspond
+// to whether the pseudo-element is tree-like, but we don't support these
+// pseudo-classes on ::before and ::after generated content yet.  See
+// http://dev.w3.org/csswg/selectors4/#pseudo-elements.
+#define CSS_PSEUDO_ELEMENT_SUPPORTS_USER_ACTION_STATE (1 << 3)
+// Should this pseudo-element be enabled only for UA sheets?
+#define CSS_PSEUDO_ELEMENT_ENABLED_IN_UA_SHEETS (1 << 4)
+// Should this pseudo-element be enabled only for UA sheets and chrome
+// stylesheets?
+#define CSS_PSEUDO_ELEMENT_ENABLED_IN_CHROME (1 << 5)
+
+#define CSS_PSEUDO_ELEMENT_ENABLED_IN_UA_SHEETS_AND_CHROME \
+  (CSS_PSEUDO_ELEMENT_ENABLED_IN_UA_SHEETS |               \
+   CSS_PSEUDO_ELEMENT_ENABLED_IN_CHROME)
+
+// Can we use the ChromeOnly document.createElement(..., { pseudo: "::foo" })
+// API for creating pseudo-implementing native anonymous content in JS with this
+// pseudo-element?
+#define CSS_PSEUDO_ELEMENT_IS_JS_CREATED_NAC (1 << 6)
+// Does this pseudo-element act like an item for containers (such as flex and
+// grid containers) and thus needs parent display-based style fixup?
+#define CSS_PSEUDO_ELEMENT_IS_FLEX_OR_GRID_ITEM (1 << 7)
+
+class nsCSSPseudoElements {
+  typedef mozilla::PseudoStyleType Type;
+  typedef mozilla::CSSEnabledState EnabledState;
+
+ public:
+  static bool IsEagerlyCascadedInServo(const Type aType) {
+    return PseudoElementHasFlags(aType, CSS_PSEUDO_ELEMENT_IS_CSS2);
+  }
+
+ public:
+#ifdef DEBUG
+  static void AssertAtoms();
+#endif
+
+// Alias nsCSSPseudoElements::foo() to nsGkAtoms::foo.
+#define CSS_PSEUDO_ELEMENT(name_, value_, flags_)         \
+  static nsCSSPseudoElementStaticAtom* name_() {          \
+    return const_cast<nsCSSPseudoElementStaticAtom*>(     \
+        static_cast<const nsCSSPseudoElementStaticAtom*>( \
+            nsGkAtoms::PseudoElement_##name_));           \
+  }
+#include "nsCSSPseudoElementList.h"
+#undef CSS_PSEUDO_ELEMENT
+
+  // Returns an empty tuple for a syntactically invalid pseudo-element, and
+  // NotPseudo for the empty / null string.
+  // The second element of the tuple (functional pseudo parameter) is currently
+  // only used for `::highlight()` pseudos and is `nullptr` otherwise.
+  static std::tuple<mozilla::Maybe<Type>, RefPtr<nsAtom>> ParsePseudoElement(
+      const nsAString& aPseudoElement,
+      EnabledState = EnabledState::ForAllContent);
+
+  // Returns Nothing() for a syntactically invalid pseudo-element, and NotPseudo
+  // for the empty / null string.
+  static mozilla::Maybe<Type> GetPseudoType(
+      const nsAString& aPseudoElement,
+      EnabledState = EnabledState::ForAllContent);
+
+  // Get the atom for a given Type. aType must be <
+  // PseudoType::CSSPseudoElementsEnd.
+  // This only ever returns static atoms, so it's fine to return a raw pointer.
+  static nsAtom* GetPseudoAtom(Type aType);
+
+  // Get the atom for a given pseudo-element string (e.g. "::before").  This can
+  // return dynamic atoms, for unrecognized pseudo-elements.
+  static already_AddRefed<nsAtom> GetPseudoAtom(
+      const nsAString& aPseudoElement);
+
+  static bool PseudoElementContainsElements(const Type aType) {
+    return PseudoElementHasFlags(aType, CSS_PSEUDO_ELEMENT_CONTAINS_ELEMENTS);
+  }
+
+  static bool PseudoElementSupportsStyleAttribute(const Type aType) {
+    MOZ_ASSERT(aType < Type::CSSPseudoElementsEnd);
+    return PseudoElementHasFlags(aType,
+                                 CSS_PSEUDO_ELEMENT_SUPPORTS_STYLE_ATTRIBUTE);
+  }
+
+  static bool PseudoElementSupportsUserActionState(const Type aType);
+
+  static bool PseudoElementIsJSCreatedNAC(Type aType) {
+    return PseudoElementHasFlags(aType, CSS_PSEUDO_ELEMENT_IS_JS_CREATED_NAC);
+  }
+
+  static bool PseudoElementIsFlexOrGridItem(const Type aType) {
+    return PseudoElementHasFlags(aType,
+                                 CSS_PSEUDO_ELEMENT_IS_FLEX_OR_GRID_ITEM);
+  }
+
+  static bool EnabledInContent(Type aType) {
+    switch (aType) {
+      case Type::highlight:
+        return mozilla::StaticPrefs::dom_customHighlightAPI_enabled();
+      case Type::sliderTrack:
+      case Type::sliderThumb:
+      case Type::sliderFill:
+        return mozilla::StaticPrefs::layout_css_modern_range_pseudos_enabled();
+      default:
+        return !PseudoElementHasAnyFlag(
+            aType, CSS_PSEUDO_ELEMENT_ENABLED_IN_UA_SHEETS_AND_CHROME);
+    }
+  }
+
+  static bool IsEnabled(Type aType, EnabledState aEnabledState) {
+    if (EnabledInContent(aType)) {
+      return true;
+    }
+
+    if ((aEnabledState & EnabledState::InUASheets) &&
+        PseudoElementHasFlags(aType, CSS_PSEUDO_ELEMENT_ENABLED_IN_UA_SHEETS)) {
+      return true;
+    }
+
+    if ((aEnabledState & EnabledState::InChrome) &&
+        PseudoElementHasFlags(aType, CSS_PSEUDO_ELEMENT_ENABLED_IN_CHROME)) {
+      return true;
+    }
+
+    return false;
+  }
+
+  static nsString PseudoTypeAsString(Type aPseudoType);
+
+ private:
+  // Does the given pseudo-element have all of the flags given?
+  static bool PseudoElementHasFlags(const Type aType, uint32_t aFlags) {
+    MOZ_ASSERT(aType < Type::CSSPseudoElementsEnd);
+    return (kPseudoElementFlags[size_t(aType)] & aFlags) == aFlags;
+  }
+
+  static bool PseudoElementHasAnyFlag(const Type aType, uint32_t aFlags) {
+    MOZ_ASSERT(aType < Type::CSSPseudoElementsEnd);
+    return (kPseudoElementFlags[size_t(aType)] & aFlags) != 0;
+  }
+
+  static nsStaticAtom* GetAtomBase();
+
+  static const uint32_t kPseudoElementFlags[size_t(Type::CSSPseudoElementsEnd)];
+};
+
+#endif /* nsCSSPseudoElements_h___ */