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diff --git a/servo/components/style/gecko/pseudo_element.rs b/servo/components/style/gecko/pseudo_element.rs
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+++ b/servo/components/style/gecko/pseudo_element.rs
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+/* 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 https://mozilla.org/MPL/2.0/. */
+
+//! Gecko's definition of a pseudo-element.
+//!
+//! Note that a few autogenerated bits of this live in
+//! `pseudo_element_definition.mako.rs`. If you touch that file, you probably
+//! need to update the checked-in files for Servo.
+
+use crate::gecko_bindings::structs::{self, PseudoStyleType};
+use crate::properties::longhands::display::computed_value::T as Display;
+use crate::properties::{ComputedValues, PropertyFlags};
+use crate::selector_parser::{PseudoElementCascadeType, SelectorImpl};
+use crate::str::{starts_with_ignore_ascii_case, string_as_ascii_lowercase};
+use crate::string_cache::Atom;
+use crate::values::serialize_atom_identifier;
+use crate::values::AtomIdent;
+use cssparser::ToCss;
+use static_prefs::pref;
+use std::fmt;
+
+include!(concat!(
+    env!("OUT_DIR"),
+    "/gecko/pseudo_element_definition.rs"
+));
+
+impl ::selectors::parser::PseudoElement for PseudoElement {
+    type Impl = SelectorImpl;
+
+    // ::slotted() should support all tree-abiding pseudo-elements, see
+    // https://drafts.csswg.org/css-scoping/#slotted-pseudo
+    // https://drafts.csswg.org/css-pseudo-4/#treelike
+    #[inline]
+    fn valid_after_slotted(&self) -> bool {
+        matches!(
+            *self,
+            Self::Before |
+                Self::After |
+                Self::Marker |
+                Self::Placeholder |
+                Self::FileSelectorButton
+        )
+    }
+
+    #[inline]
+    fn accepts_state_pseudo_classes(&self) -> bool {
+        self.supports_user_action_state()
+    }
+}
+
+impl PseudoElement {
+    /// Returns the kind of cascade type that a given pseudo is going to use.
+    ///
+    /// In Gecko we only compute ::before and ::after eagerly. We save the rules
+    /// for anonymous boxes separately, so we resolve them as precomputed
+    /// pseudos.
+    ///
+    /// We resolve the others lazily, see `Servo_ResolvePseudoStyle`.
+    pub fn cascade_type(&self) -> PseudoElementCascadeType {
+        if self.is_eager() {
+            debug_assert!(!self.is_anon_box());
+            return PseudoElementCascadeType::Eager;
+        }
+
+        if self.is_precomputed() {
+            return PseudoElementCascadeType::Precomputed;
+        }
+
+        PseudoElementCascadeType::Lazy
+    }
+
+    /// Gets the canonical index of this eagerly-cascaded pseudo-element.
+    #[inline]
+    pub fn eager_index(&self) -> usize {
+        EAGER_PSEUDOS
+            .iter()
+            .position(|p| p == self)
+            .expect("Not an eager pseudo")
+    }
+
+    /// Creates a pseudo-element from an eager index.
+    #[inline]
+    pub fn from_eager_index(i: usize) -> Self {
+        EAGER_PSEUDOS[i].clone()
+    }
+
+    /// Whether animations for the current pseudo element are stored in the
+    /// parent element.
+    #[inline]
+    pub fn animations_stored_in_parent(&self) -> bool {
+        matches!(*self, Self::Before | Self::After | Self::Marker)
+    }
+
+    /// Whether the current pseudo element is ::before or ::after.
+    #[inline]
+    pub fn is_before_or_after(&self) -> bool {
+        matches!(*self, Self::Before | Self::After)
+    }
+
+    /// Whether this pseudo-element is the ::before pseudo.
+    #[inline]
+    pub fn is_before(&self) -> bool {
+        *self == PseudoElement::Before
+    }
+
+    /// Whether this pseudo-element is the ::after pseudo.
+    #[inline]
+    pub fn is_after(&self) -> bool {
+        *self == PseudoElement::After
+    }
+
+    /// Whether this pseudo-element is the ::marker pseudo.
+    #[inline]
+    pub fn is_marker(&self) -> bool {
+        *self == PseudoElement::Marker
+    }
+
+    /// Whether this pseudo-element is the ::selection pseudo.
+    #[inline]
+    pub fn is_selection(&self) -> bool {
+        *self == PseudoElement::Selection
+    }
+
+    /// Whether this pseudo-element is ::first-letter.
+    #[inline]
+    pub fn is_first_letter(&self) -> bool {
+        *self == PseudoElement::FirstLetter
+    }
+
+    /// Whether this pseudo-element is ::first-line.
+    #[inline]
+    pub fn is_first_line(&self) -> bool {
+        *self == PseudoElement::FirstLine
+    }
+
+    /// Whether this pseudo-element is the ::-moz-color-swatch pseudo.
+    #[inline]
+    pub fn is_color_swatch(&self) -> bool {
+        *self == PseudoElement::MozColorSwatch
+    }
+
+    /// Whether this pseudo-element is lazily-cascaded.
+    #[inline]
+    pub fn is_lazy(&self) -> bool {
+        !self.is_eager() && !self.is_precomputed()
+    }
+
+    /// The identifier of the highlight this pseudo-element represents.
+    pub fn highlight_name(&self) -> Option<&AtomIdent> {
+        match *self {
+            Self::Highlight(ref name) => Some(name),
+            _ => None,
+        }
+    }
+
+    /// Whether this pseudo-element is the ::highlight pseudo.
+    pub fn is_highlight(&self) -> bool {
+        matches!(*self, Self::Highlight(_))
+    }
+
+    /// Whether this pseudo-element supports user action selectors.
+    pub fn supports_user_action_state(&self) -> bool {
+        (self.flags() & structs::CSS_PSEUDO_ELEMENT_SUPPORTS_USER_ACTION_STATE) != 0
+    }
+
+    /// Whether this pseudo-element is enabled for all content.
+    pub fn enabled_in_content(&self) -> bool {
+        match *self {
+            Self::Highlight(..) => pref!("dom.customHighlightAPI.enabled"),
+            Self::SliderFill | Self::SliderTrack | Self::SliderThumb => {
+                pref!("layout.css.modern-range-pseudos.enabled")
+            },
+            // If it's not explicitly enabled in UA sheets or chrome, then we're enabled for
+            // content.
+            _ => (self.flags() & structs::CSS_PSEUDO_ELEMENT_ENABLED_IN_UA_SHEETS_AND_CHROME) == 0,
+        }
+    }
+
+    /// Whether this pseudo is enabled explicitly in UA sheets.
+    pub fn enabled_in_ua_sheets(&self) -> bool {
+        (self.flags() & structs::CSS_PSEUDO_ELEMENT_ENABLED_IN_UA_SHEETS) != 0
+    }
+
+    /// Whether this pseudo is enabled explicitly in chrome sheets.
+    pub fn enabled_in_chrome(&self) -> bool {
+        (self.flags() & structs::CSS_PSEUDO_ELEMENT_ENABLED_IN_CHROME) != 0
+    }
+
+    /// Whether this pseudo-element skips flex/grid container display-based
+    /// fixup.
+    #[inline]
+    pub fn skip_item_display_fixup(&self) -> bool {
+        (self.flags() & structs::CSS_PSEUDO_ELEMENT_IS_FLEX_OR_GRID_ITEM) == 0
+    }
+
+    /// Whether this pseudo-element is precomputed.
+    #[inline]
+    pub fn is_precomputed(&self) -> bool {
+        self.is_anon_box() && !self.is_tree_pseudo_element()
+    }
+
+    /// Property flag that properties must have to apply to this pseudo-element.
+    #[inline]
+    pub fn property_restriction(&self) -> Option<PropertyFlags> {
+        Some(match *self {
+            PseudoElement::FirstLetter => PropertyFlags::APPLIES_TO_FIRST_LETTER,
+            PseudoElement::FirstLine => PropertyFlags::APPLIES_TO_FIRST_LINE,
+            PseudoElement::Placeholder => PropertyFlags::APPLIES_TO_PLACEHOLDER,
+            PseudoElement::Cue => PropertyFlags::APPLIES_TO_CUE,
+            PseudoElement::Marker if static_prefs::pref!("layout.css.marker.restricted") => {
+                PropertyFlags::APPLIES_TO_MARKER
+            },
+            _ => return None,
+        })
+    }
+
+    /// Whether this pseudo-element should actually exist if it has
+    /// the given styles.
+    pub fn should_exist(&self, style: &ComputedValues) -> bool {
+        debug_assert!(self.is_eager());
+
+        if style.get_box().clone_display() == Display::None {
+            return false;
+        }
+
+        if self.is_before_or_after() && style.ineffective_content_property() {
+            return false;
+        }
+
+        true
+    }
+}