LibWeb/CSS: Add basic registered properties with initial values

Add global registry for registered properties and partial support
for `@property` rule. Enables registering properties with initial
values. Also adds basic retrieval via `var()`.

Note: This is not a complete `@property` implementation.

Libraries/LibWeb/CSS/CSSPropertyRule.h

@@ -29,6 +29,7 @@ public:
     FlyString const& syntax() const { return m_syntax; }
     bool inherits() const { return m_inherits; }
     Optional<String> initial_value() const;
+    RefPtr<CSSStyleValue const> initial_style_value() const { return m_initial_value; }
 
 private:
     CSSPropertyRule(JS::Realm&, FlyString name, FlyString syntax, bool inherits, RefPtr<CSSStyleValue const> initial_value);

Libraries/LibWeb/CSS/Parser/Parser.cpp

@@ -145,6 +145,13 @@ GC::RootVector<GC::Ref<CSSRule>> Parser::convert_rules(Vector<Rule> const& raw_r
 
             m_declared_namespaces.set(as<CSSNamespaceRule>(*rule).prefix());
             break;
+        case CSSRule::Type::Property: {
+            auto& property_rule = as<CSSPropertyRule>(*rule);
+            if (m_document) {
+                const_cast<DOM::Document*>(m_document.ptr())->registered_custom_properties().set(property_rule.name(), property_rule);
+            }
+            [[fallthrough]];
+        }
         default:
             import_rules_valid = false;
             namespace_rules_valid = false;

Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -3106,33 +3106,25 @@ void StyleComputer::unload_fonts_from_sheet(CSSStyleSheet& sheet)
     }
 }
 
-static NonnullRefPtr<CSSStyleValue const> custom_property_initial_value(FlyString const& name)
-{
-    // FIXME: Look-up initial value for registered properties. (@property)
-    (void)name;
-
-    // For non-registered properties, the initial value is the guaranteed-invalid value.
-    return GuaranteedInvalidStyleValue::create();
-}
-
 NonnullRefPtr<CSSStyleValue const> StyleComputer::compute_value_of_custom_property(DOM::AbstractElement abstract_element, FlyString const& name, Optional<Parser::GuardedSubstitutionContexts&> guarded_contexts)
 {
     // https://drafts.csswg.org/css-variables/#propdef-
     // The computed value of a custom property is its specified value with any arbitrary-substitution functions replaced.
     // FIXME: These should probably be part of ComputedProperties.
+    auto& document = abstract_element.document();
 
     auto value = abstract_element.get_custom_property(name);
     if (!value || value->is_initial())
-        return custom_property_initial_value(name);
+        return document.custom_property_initial_value(name);
 
     // Unset is the same as inherit for inherited properties, and by default all custom properties are inherited.
     // FIXME: Support non-inherited registered custom properties.
     if (value->is_inherit() || value->is_unset()) {
         if (!abstract_element.parent_element())
-            return custom_property_initial_value(name);
+            return document.custom_property_initial_value(name);
         auto inherited_value = DOM::AbstractElement { const_cast<DOM::Element&>(*abstract_element.parent_element()) }.get_custom_property(name);
         if (!inherited_value)
-            return custom_property_initial_value(name);
+            return document.custom_property_initial_value(name);
         return inherited_value.release_nonnull();
     }
 

Libraries/LibWeb/DOM/Document.cpp

@@ -46,6 +46,7 @@
 #include <LibWeb/CSS/StyleComputer.h>
 #include <LibWeb/CSS/StyleSheetIdentifier.h>
 #include <LibWeb/CSS/StyleValues/ColorSchemeStyleValue.h>
+#include <LibWeb/CSS/StyleValues/GuaranteedInvalidStyleValue.h>
 #include <LibWeb/CSS/SystemColor.h>
 #include <LibWeb/CSS/TransitionEvent.h>
 #include <LibWeb/CSS/VisualViewport.h>
@@ -620,6 +621,7 @@ void Document::visit_edges(Cell::Visitor& visitor)
     visitor.visit(m_render_blocking_elements);
     visitor.visit(m_policy_container);
     visitor.visit(m_style_invalidator);
+    visitor.visit(m_registered_custom_properties);
 }
 
 // https://w3c.github.io/selection-api/#dom-document-getselection
@@ -6576,6 +6578,26 @@ String Document::dump_display_list()
     return display_list->dump();
 }
 
+HashMap<FlyString, GC::Ref<Web::CSS::CSSPropertyRule>>& Document::registered_custom_properties()
+{
+    return m_registered_custom_properties;
+}
+
+NonnullRefPtr<CSS::CSSStyleValue const> Document::custom_property_initial_value(FlyString const& name) const
+{
+    auto maybe_custom_property = m_registered_custom_properties.get(name);
+    if (maybe_custom_property.has_value()) {
+        auto parsed_value = maybe_custom_property.value()->initial_style_value();
+        if (!parsed_value)
+            return CSS::GuaranteedInvalidStyleValue::create();
+        return parsed_value.release_nonnull();
+    }
+
+    // For non-registered properties, the initial value is the guaranteed-invalid value.
+    // See: https://drafts.csswg.org/css-variables/#propdef-
+    return CSS::GuaranteedInvalidStyleValue::create();
+}
+
 GC::Ptr<Element> ElementByIdMap::get(FlyString const& element_id) const
 {
     if (auto elements = m_map.get(element_id); elements.has_value() && !elements->is_empty()) {

Libraries/LibWeb/DOM/Document.h

@@ -21,6 +21,7 @@
 #include <LibURL/Origin.h>
 #include <LibURL/URL.h>
 #include <LibUnicode/Forward.h>
+#include <LibWeb/CSS/CSSPropertyRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
 #include <LibWeb/CSS/StyleSheetList.h>
 #include <LibWeb/Cookie/Cookie.h>
@@ -905,6 +906,11 @@ public:
 
     StyleInvalidator& style_invalidator() { return m_style_invalidator; }
 
+    // https://www.w3.org/TR/css-properties-values-api-1/#dom-window-registeredpropertyset-slot
+    HashMap<FlyString, GC::Ref<Web::CSS::CSSPropertyRule>>& registered_custom_properties();
+
+    NonnullRefPtr<CSS::CSSStyleValue const> custom_property_initial_value(FlyString const& name) const;
+
 protected:
     virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
@@ -1265,6 +1271,9 @@ private:
     HashTable<WeakPtr<Node>> m_pending_nodes_for_style_invalidation_due_to_presence_of_has;
 
     GC::Ref<StyleInvalidator> m_style_invalidator;
+
+    // https://www.w3.org/TR/css-properties-values-api-1/#dom-window-registeredpropertyset-slot
+    HashMap<FlyString, GC::Ref<Web::CSS::CSSPropertyRule>> m_registered_custom_properties;
 };
 
 template<>

Tests/LibWeb/Text/expected/css/at-property-initial-value-background-color.txt

@@ -0,0 +1 @@
+Computed background-color: rgb(255, 0, 0)

Tests/LibWeb/Text/expected/wpt-import/css/css-values/attr-all-types.txt

@@ -2,8 +2,8 @@ Harness status: OK
 
 Found 139 tests
 
-128 Pass
-11 Fail
+130 Pass
+9 Fail
 Pass	CSS Values and Units Test: attr
 Fail	CSS Values and Units Test: attr 1
 Pass	CSS Values and Units Test: attr 2
@@ -54,12 +54,12 @@ Pass	CSS Values and Units Test: attr 46
 Pass	CSS Values and Units Test: attr 47
 Pass	CSS Values and Units Test: attr 48
 Pass	CSS Values and Units Test: attr 49
-Fail	CSS Values and Units Test: attr 50
+Pass	CSS Values and Units Test: attr 50
 Fail	CSS Values and Units Test: attr 51
 Pass	CSS Values and Units Test: attr 52
 Pass	CSS Values and Units Test: attr 53
 Pass	CSS Values and Units Test: attr 54
-Fail	CSS Values and Units Test: attr 55
+Pass	CSS Values and Units Test: attr 55
 Pass	CSS Values and Units Test: attr 56
 Pass	CSS Values and Units Test: attr 57
 Pass	CSS Values and Units Test: attr 58

Tests/LibWeb/Text/expected/wpt-import/css/css-variables/variable-css-wide-keywords.txt

@@ -2,15 +2,15 @@ Harness status: OK
 
 Found 30 tests
 
-9 Pass
-21 Fail
+11 Pass
+19 Fail
 Pass	`initial` as a value for an unregistered custom property
 Pass	`inherit` as a value for an unregistered custom property
 Pass	`unset` as a value for an unregistered custom property
 Fail	`revert` as a value for an unregistered custom property
 Pass	`revert-layer` as a value for an unregistered custom property
-Fail	`initial` as a value for a non-inheriting registered custom property
-Fail	`initial` as a value for an inheriting registered custom property
+Pass	`initial` as a value for a non-inheriting registered custom property
+Pass	`initial` as a value for an inheriting registered custom property
 Pass	`inherit` as a value for a non-inheriting registered custom property
 Pass	`inherit` as a value for an inheriting registered custom property
 Fail	`unset` as a value for a non-inheriting registered custom property

Tests/LibWeb/Text/input/css/at-property-initial-value-background-color.html

@@ -0,0 +1,21 @@
+<!DOCTYPE html>
+<style>
+    .background {
+        background-color: var(--bg-color);
+    }
+
+    @property --bg-color {
+        syntax: '*';
+        inherits: false;
+        initial-value: red;
+    }
+</style>
+<div class="background">It should have red bg</div>
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const el = document.querySelector(".background");
+        const style = getComputedStyle(el);
+        println(`Computed background-color: ${style.backgroundColor}`);
+    });
+</script>