LibWeb: Add CSS rule buckets for pseudo elements, and for :root

If a selector must match a pseudo element, or must match the root
element, we now cache that information in the MatchingRule struct.
We also introduce separate buckets for these rules, so we can avoid
running them altogether if the current element can't possibly match.

This cuts the number of selectors evaluated by 32% when loading our
GitHub repo page https://github.com/SerenityOS/serenity

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -336,6 +336,10 @@ Vector<MatchingRule> StyleComputer::collect_matching_rules(DOM::Element const& e
     }
     if (auto it = rule_cache.rules_by_tag_name.find(element.local_name()); it != rule_cache.rules_by_tag_name.end())
         add_rules_to_run(it->value);
+    if (pseudo_element.has_value())
+        add_rules_to_run(rule_cache.pseudo_element_rules);
+    if (element.is_document_element())
+        add_rules_to_run(rule_cache.root_rules);
     add_rules_to_run(rule_cache.other_rules);
 
     Vector<MatchingRule> matching_rules;
@@ -2290,6 +2294,7 @@ NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_casca
     size_t num_id_rules = 0;
     size_t num_tag_name_rules = 0;
     size_t num_pseudo_element_rules = 0;
+    size_t num_root_rules = 0;
 
     Vector<MatchingRule> matching_rules;
     size_t style_sheet_index = 0;
@@ -2310,10 +2315,18 @@ NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_casca
                 };
 
                 for (auto const& simple_selector : selector.compound_selectors().last().simple_selectors) {
-                    if (simple_selector.type == CSS::Selector::SimpleSelector::Type::PseudoElement) {
-                        matching_rule.contains_pseudo_element = true;
-                        ++num_pseudo_element_rules;
-                        break;
+                    if (!matching_rule.contains_pseudo_element) {
+                        if (simple_selector.type == CSS::Selector::SimpleSelector::Type::PseudoElement) {
+                            matching_rule.contains_pseudo_element = true;
+                            ++num_pseudo_element_rules;
+                        }
+                    }
+                    if (!matching_rule.contains_root_pseudo_class) {
+                        if (simple_selector.type == CSS::Selector::SimpleSelector::Type::PseudoClass
+                            && simple_selector.pseudo_class().type == CSS::PseudoClass::Root) {
+                            matching_rule.contains_root_pseudo_class = true;
+                            ++num_root_rules;
+                        }
                     }
                 }
 
@@ -2338,8 +2351,14 @@ NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_casca
                         break;
                     }
                 }
-                if (!added_to_bucket)
-                    rule_cache->other_rules.append(move(matching_rule));
+                if (!added_to_bucket) {
+                    if (matching_rule.contains_pseudo_element)
+                        rule_cache->pseudo_element_rules.append(move(matching_rule));
+                    else if (matching_rule.contains_root_pseudo_class)
+                        rule_cache->root_rules.append(move(matching_rule));
+                    else
+                        rule_cache->other_rules.append(move(matching_rule));
+                }
 
                 ++selector_index;
             }
@@ -2391,6 +2410,7 @@ NonnullOwnPtr<StyleComputer::RuleCache> StyleComputer::make_rule_cache_for_casca
         dbgln("        Class: {}", num_class_rules);
         dbgln("      TagName: {}", num_tag_name_rules);
         dbgln("PseudoElement: {}", num_pseudo_element_rules);
+        dbgln("         Root: {}", num_root_rules);
         dbgln("        Other: {}", rule_cache->other_rules.size());
         dbgln("        Total: {}", num_class_rules + num_id_rules + num_tag_name_rules + rule_cache->other_rules.size());
     }

Userland/Libraries/LibWeb/CSS/StyleComputer.h

@@ -37,8 +37,10 @@ struct MatchingRule {
     size_t style_sheet_index { 0 };
     size_t rule_index { 0 };
     size_t selector_index { 0 };
+
     u32 specificity { 0 };
     bool contains_pseudo_element { false };
+    bool contains_root_pseudo_class { false };
 };
 
 struct FontFaceKey {
@@ -129,6 +131,8 @@ class StyleComputer {
         HashMap<FlyString, Vector<MatchingRule>> rules_by_id;
         HashMap<FlyString, Vector<MatchingRule>> rules_by_class;
         HashMap<FlyString, Vector<MatchingRule>> rules_by_tag_name;
+        Vector<MatchingRule> pseudo_element_rules;
+        Vector<MatchingRule> root_rules;
         Vector<MatchingRule> other_rules;
 
         HashMap<FlyString, NonnullRefPtr<Animations::KeyframeEffect::KeyFrameSet>> rules_by_animation_keyframes;