LibHTML: Implement some very simple selector matching.

We walk the entire DOM and check all selectors against all elements. Only
id, class and tag name are checked right now. There's no ancestor stack
or compound selectors. All in good time :^)

LibHTML/CSS/StyleResolver.cpp

@@ -1,5 +1,8 @@
 #include <LibHTML/CSS/StyleResolver.h>
 #include <LibHTML/CSS/StyleSheet.h>
+#include <LibHTML/DOM/Element.h>
+#include <LibHTML/Dump.h>
+#include <stdio.h>
 
 StyleResolver::StyleResolver(Document& document)
     : m_document(document)
@@ -10,14 +13,53 @@ StyleResolver::~StyleResolver()
 {
 }
 
+static bool matches(const Selector& selector, const Element& element)
+{
+    // FIXME: Support compound selectors.
+    ASSERT(selector.components().size() == 1);
+
+    auto& component = selector.components().first();
+    switch (component.type) {
+    case Selector::Component::Type::Id:
+        return component.value == element.attribute("id");
+    case Selector::Component::Type::Class:
+        return element.has_class(component.value);
+    case Selector::Component::Type::TagName:
+        return component.value == element.tag_name();
+    default:
+        ASSERT_NOT_REACHED();
+    }
+}
+
+NonnullRefPtrVector<StyleRule> StyleResolver::collect_matching_rules(const Element& element) const
+{
+    NonnullRefPtrVector<StyleRule> matching_rules;
+    for (auto& sheet : m_sheets) {
+        for (auto& rule : sheet.rules()) {
+            for (auto& selector : rule.selectors()) {
+                if (matches(selector, element)) {
+                    matching_rules.append(rule);
+                    break;
+                }
+            }
+        }
+    }
+    printf("Rules matching Element{%p}\n", &element);
+    for (auto& rule : matching_rules) {
+        dump_rule(rule);
+    }
+    return matching_rules;
+}
+
 OwnPtr<LayoutStyle> StyleResolver::resolve_document_style(const Document& document)
 {
     UNUSED_PARAM(document);
-    return nullptr;
+    return make<LayoutStyle>();
 }
 
 OwnPtr<LayoutStyle> StyleResolver::resolve_element_style(const Element& element)
 {
-    UNUSED_PARAM(element);
-    return nullptr;
+    auto style = make<LayoutStyle>();
+    auto matching_rules = collect_matching_rules(element);
+    return style;
 }