LibHTML: Add is<ElementType> and to<ElementType> helper functions

These will help us write node-type-aware template functions.

Libraries/LibHTML/DOM/Document.cpp

@@ -29,11 +29,8 @@ StyleResolver& Document::style_resolver()
 
 void Document::normalize()
 {
-    if (first_child() != nullptr && first_child()->is_element()) {
-        const Element& el = static_cast<const Element&>(*first_child());
-        if (el.tag_name() == "html")
-            return;
-    }
+    if (is<HTMLHtmlElement>(first_child()))
+        return;
 
     NonnullRefPtr<Element> body = adopt(*new Element(*this, "body"));
     NonnullRefPtr<Element> html = adopt(*new Element(*this, "html"));

Libraries/LibHTML/DOM/Document.h

@@ -80,3 +80,9 @@ private:
     Color m_active_link_color { Color::Red };
     Color m_visited_link_color { Color::Magenta };
 };
+
+template<>
+inline bool is<Document>(const Node& node)
+{
+    return node.is_document();
+}

Libraries/LibHTML/DOM/Element.h

@@ -26,7 +26,7 @@ public:
     Element(Document&, const String& tag_name);
     virtual ~Element() override;
 
-    virtual String tag_name() const override { return m_tag_name; }
+    virtual String tag_name() const final { return m_tag_name; }
 
     String attribute(const String& name) const;
     void set_attribute(const String& name, const String& value);
@@ -54,3 +54,9 @@ private:
     String m_tag_name;
     Vector<Attribute> m_attributes;
 };
+
+template<>
+inline bool is<Element>(const Node& node)
+{
+    return node.is_element();
+}

Libraries/LibHTML/DOM/HTMLAnchorElement.h

@@ -9,3 +9,9 @@ public:
 
     String href() const { return attribute("href"); }
 };
+
+template<>
+inline bool is<HTMLAnchorElement>(const Node& node)
+{
+    return is<Element>(node) && to<Element>(node).tag_name().to_lowercase() == "a";
+}

Libraries/LibHTML/DOM/HTMLElement.h

@@ -12,3 +12,9 @@ public:
 private:
     virtual bool is_html_element() const final { return true; }
 };
+
+template<>
+inline bool is<HTMLElement>(const Node& node)
+{
+    return node.is_html_element();
+}

Libraries/LibHTML/DOM/HTMLHtmlElement.h

@@ -7,3 +7,9 @@ public:
     HTMLHtmlElement(Document&, const String& tag_name);
     virtual ~HTMLHtmlElement() override;
 };
+
+template<>
+inline bool is<HTMLHtmlElement>(const Node& node)
+{
+    return is<Element>(node) && to<Element>(node).tag_name().to_lowercase() == "html";
+}

Libraries/LibHTML/DOM/Node.cpp

@@ -56,7 +56,7 @@ RefPtr<LayoutNode> Node::create_layout_tree(const StyleResolver& resolver, const
 
 const HTMLAnchorElement* Node::enclosing_link_element() const
 {
-    if (is_element() && tag_name().to_lowercase() == "a")
+    if (is<HTMLAnchorElement>(*this))
         return static_cast<const HTMLAnchorElement*>(this);
     return parent() ? parent()->enclosing_link_element() : nullptr;
 }

Libraries/LibHTML/DOM/Node.h

@@ -74,3 +74,41 @@ protected:
     mutable LayoutNode* m_layout_node { nullptr };
     NodeType m_type { NodeType::INVALID };
 };
+
+template<typename T>
+inline bool is(const Node&)
+{
+    return false;
+}
+
+template<typename T>
+inline bool is(const Node* node)
+{
+    return node && is<T>(*node);
+}
+
+template<>
+inline bool is<Node>(const Node&)
+{
+    return true;
+}
+
+template<>
+inline bool is<ParentNode>(const Node& node)
+{
+    return node.is_parent_node();
+}
+
+template<typename T>
+inline const T& to(const Node& node)
+{
+    ASSERT(is<T>(node));
+    return static_cast<const T&>(node);
+}
+
+template<typename T>
+inline T& to(Node& node)
+{
+    ASSERT(is<T>(node));
+    return static_cast<T&>(node);
+}

Libraries/LibHTML/DOM/Text.h

@@ -19,3 +19,9 @@ private:
 
     String m_data;
 };
+
+template<>
+inline bool is<Text>(const Node& node)
+{
+    return node.is_text();
+}