LibWeb: Speed up CSS namespace checking

CSSStyleSheet now caches the CSSNamespaceRule for the default namespace,
which is the only one we currently care about. This saves us from
iterating over its list of rules every time we want to know what that
default namespace is.

The spec dictates that `@namespace` rules are only valid near the start
of a stylesheet, so we also take advantage of that to quit searching
for namespaces as soon as we see a non-import rule.

Also renamed `namespace_filter()` to `default_namespace()` since that's
what it actually returns.

This makes github.com/serenityos/serenity snappy again. :^)

Userland/Libraries/LibWeb/CSS/CSSStyleSheet.cpp

@@ -6,7 +6,6 @@
 
 #include <LibWeb/Bindings/CSSStyleSheetPrototype.h>
 #include <LibWeb/Bindings/Intrinsics.h>
-#include <LibWeb/CSS/CSSNamespaceRule.h>
 #include <LibWeb/CSS/CSSStyleSheet.h>
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/CSS/StyleComputer.h>
@@ -30,6 +29,12 @@ CSSStyleSheet::CSSStyleSheet(JS::Realm& realm, CSSRuleList& rules, MediaList& me
 
     for (auto& rule : *m_rules)
         rule->set_parent_style_sheet(this);
+
+    recalculate_namespaces();
+
+    m_rules->on_change = [this]() {
+        recalculate_namespaces();
+    };
 }
 
 JS::ThrowCompletionOr<void> CSSStyleSheet::initialize(JS::Realm& realm)
@@ -138,17 +143,40 @@ void CSSStyleSheet::set_style_sheet_list(Badge<StyleSheetList>, StyleSheetList*
     m_style_sheet_list = list;
 }
 
-Optional<StringView> CSSStyleSheet::namespace_filter() const
+Optional<StringView> CSSStyleSheet::default_namespace() const
+{
+    if (m_default_namespace_rule)
+        return m_default_namespace_rule->namespace_uri().view();
+
+    return {};
+}
+
+void CSSStyleSheet::recalculate_namespaces()
 {
     for (JS::NonnullGCPtr<CSSRule> rule : *m_rules) {
-        if (rule->type() == CSSRule::Type::Namespace) {
-            auto& namespace_rule = verify_cast<CSSNamespaceRule>(*rule);
-            if (!namespace_rule.namespace_uri().is_empty() && namespace_rule.prefix().is_empty())
-                return namespace_rule.namespace_uri().view();
+        // "Any @namespace rules must follow all @charset and @import rules and precede all other
+        // non-ignored at-rules and style rules in a style sheet.
+        // ...
+        // A syntactically invalid @namespace rule (whether malformed or misplaced) must be ignored."
+        // https://drafts.csswg.org/css-namespaces/#syntax
+        switch (rule->type()) {
+        case CSSRule::Type::Import:
+            continue;
+
+        case CSSRule::Type::Namespace:
+            break;
+
+        default:
+            // Any other types mean that further @namespace rules are invalid, so we can stop here.
+            return;
         }
-    }
 
-    return {};
+        auto& namespace_rule = verify_cast<CSSNamespaceRule>(*rule);
+        if (!namespace_rule.namespace_uri().is_empty() && namespace_rule.prefix().is_empty())
+            m_default_namespace_rule = namespace_rule;
+
+        // FIXME: Store qualified namespace rules.
+    }
 }
 
 }

Userland/Libraries/LibWeb/CSS/CSSStyleSheet.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Function.h>
+#include <LibWeb/CSS/CSSNamespaceRule.h>
 #include <LibWeb/CSS/CSSRule.h>
 #include <LibWeb/CSS/CSSRuleList.h>
 #include <LibWeb/CSS/CSSStyleRule.h>
@@ -48,7 +49,7 @@ public:
 
     void set_style_sheet_list(Badge<StyleSheetList>, StyleSheetList*);
 
-    Optional<StringView> namespace_filter() const;
+    Optional<StringView> default_namespace() const;
 
 private:
     CSSStyleSheet(JS::Realm&, CSSRuleList&, MediaList&, Optional<AK::URL> location);
@@ -56,7 +57,10 @@ private:
     virtual JS::ThrowCompletionOr<void> initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
+    void recalculate_namespaces();
+
     JS::GCPtr<CSSRuleList> m_rules;
+    JS::GCPtr<CSSNamespaceRule> m_default_namespace_rule;
 
     JS::GCPtr<StyleSheetList> m_style_sheet_list;
     JS::GCPtr<CSSRule> m_owner_css_rule;

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -225,7 +225,7 @@ Vector<MatchingRule> StyleComputer::filter_namespace_rules(DOM::Element const& e
     Vector<MatchingRule> filtered_rules;
 
     for (auto const& rule : rules) {
-        auto namespace_uri = rule.sheet->namespace_filter();
+        auto namespace_uri = rule.sheet->default_namespace();
         if (namespace_uri.has_value()) {
             if (namespace_uri.value() == element.namespace_uri())
                 filtered_rules.append(rule);