LibWeb: Split StyleComputer work into two phases with separate outputs

Before this change, StyleComputer would essentially take a DOM element,
find all the CSS rules that apply to it, and resolve the computed value
for each CSS property for that element.

This worked great, but it meant we had to do all the work of selector
matching and cascading every time.

To enable new optimizations, this change introduces a break in the
middle of this process where we've produced a "CascadedProperties".
This object contains the result of the cascade, before we've begun
turning cascaded values into computed values.

The cascaded properties are now stored with each element, which will
later allow us to do partial updates without re-running the full
StyleComputer machine. This will be particularly valuable for
re-implementing CSS inheritance, which is extremely heavy today.

Note that CSS animations and CSS transitions operate entirely on the
computed values, even though the cascade order would have you believe
they happen earlier. I'm not confident we have the right architecture
for this, but that's a separate issue.

Libraries/LibWeb/CSS/StyleComputer.h

@@ -15,6 +15,8 @@
 #include <LibWeb/CSS/CSSFontFaceRule.h>
 #include <LibWeb/CSS/CSSKeyframesRule.h>
 #include <LibWeb/CSS/CSSStyleDeclaration.h>
+#include <LibWeb/CSS/CascadeOrigin.h>
+#include <LibWeb/CSS/CascadedProperties.h>
 #include <LibWeb/CSS/Selector.h>
 #include <LibWeb/CSS/StyleProperties.h>
 #include <LibWeb/Forward.h>
@@ -71,15 +73,6 @@ private:
     CounterType m_buckets[bucket_count];
 };
 
-// https://www.w3.org/TR/css-cascade/#origin
-enum class CascadeOrigin : u8 {
-    Author,
-    User,
-    UserAgent,
-    Animation,
-    Transition,
-};
-
 struct MatchingRule {
     GC::Ptr<DOM::ShadowRoot const> shadow_root;
     GC::Ptr<CSSRule const> rule; // Either CSSStyleRule or CSSNestedDeclarations
@@ -126,7 +119,14 @@ public:
         No,
     };
     static void for_each_property_expanding_shorthands(PropertyID, CSSStyleValue const&, AllowUnresolved, Function<void(PropertyID, CSSStyleValue const&)> const& set_longhand_property);
-    static void set_property_expanding_shorthands(StyleProperties&, PropertyID, CSSStyleValue const&, CSS::CSSStyleDeclaration const*, StyleProperties const& style_for_revert, StyleProperties const& style_for_revert_layer, Important = Important::No);
+    static void set_property_expanding_shorthands(
+        CascadedProperties&,
+        PropertyID,
+        CSSStyleValue const&,
+        GC::Ptr<CSSStyleDeclaration const>,
+        CascadeOrigin,
+        Important,
+        Optional<FlyString> layer_name);
     static NonnullRefPtr<CSSStyleValue const> get_inherit_value(CSS::PropertyID, DOM::Element const*, Optional<CSS::Selector::PseudoElement::Type> = {});
 
     static Optional<String> user_agent_style_sheet_source(StringView name);
@@ -167,12 +167,14 @@ public:
         No,
         Yes,
     };
-    void collect_animation_into(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>, GC::Ref<Animations::KeyframeEffect> animation, StyleProperties& style_properties, AnimationRefresh = AnimationRefresh::No) const;
+    void collect_animation_into(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>, GC::Ref<Animations::KeyframeEffect> animation, StyleProperties&, AnimationRefresh = AnimationRefresh::No) const;
 
     [[nodiscard]] bool has_has_selectors() const { return m_has_has_selectors; }
 
     size_t number_of_css_font_faces_with_loading_in_progress() const;
 
+    [[nodiscard]] StyleProperties compute_properties(DOM::Element&, Optional<Selector::PseudoElement::Type>, CascadedProperties&) const;
+
 private:
     enum class ComputeStyleMode {
         Normal,
@@ -184,7 +186,7 @@ private:
     [[nodiscard]] bool should_reject_with_ancestor_filter(Selector const&) const;
 
     Optional<StyleProperties> compute_style_impl(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>, ComputeStyleMode) const;
-    void compute_cascaded_values(StyleProperties&, DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>, bool& did_match_any_pseudo_element_rules, ComputeStyleMode) const;
+    [[nodiscard]] GC::Ref<CascadedProperties> compute_cascaded_values(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>, bool& did_match_any_pseudo_element_rules, ComputeStyleMode) const;
     static RefPtr<Gfx::FontCascadeList const> find_matching_font_weight_ascending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, bool inclusive);
     static RefPtr<Gfx::FontCascadeList const> find_matching_font_weight_descending(Vector<MatchingFontCandidate> const& candidates, int target_weight, float font_size_in_pt, bool inclusive);
     RefPtr<Gfx::FontCascadeList const> font_matching_algorithm(FlyString const& family_name, int weight, int slope, float font_size_in_pt) const;
@@ -198,7 +200,16 @@ private:
 
     void compute_defaulted_property_value(StyleProperties&, DOM::Element const*, CSS::PropertyID, Optional<CSS::Selector::PseudoElement::Type>) const;
 
-    void set_all_properties(DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>, StyleProperties&, CSSStyleValue const&, DOM::Document&, CSS::CSSStyleDeclaration const*, StyleProperties const& style_for_revert, StyleProperties const& style_for_revert_layer, Important = Important::No) const;
+    void set_all_properties(
+        CascadedProperties&,
+        DOM::Element&,
+        Optional<Selector::PseudoElement::Type>,
+        CSSStyleValue const&,
+        DOM::Document&,
+        GC::Ptr<CSSStyleDeclaration const>,
+        CascadeOrigin,
+        Important,
+        Optional<FlyString> layer_name) const;
 
     template<typename Callback>
     void for_each_stylesheet(CascadeOrigin, Callback) const;
@@ -221,7 +232,14 @@ private:
         Vector<LayerMatchingRules> author_rules;
     };
 
-    void cascade_declarations(StyleProperties&, DOM::Element&, Optional<CSS::Selector::PseudoElement::Type>, Vector<MatchingRule> const&, CascadeOrigin, Important, StyleProperties const& style_for_revert, StyleProperties const& style_for_revert_layer) const;
+    void cascade_declarations(
+        CascadedProperties&,
+        DOM::Element&,
+        Optional<CSS::Selector::PseudoElement::Type>,
+        Vector<MatchingRule> const&,
+        CascadeOrigin,
+        Important,
+        Optional<FlyString> layer_name) const;
 
     void build_rule_cache();
     void build_rule_cache_if_needed() const;