LibWeb: Support :active pseudo-class for hyperlinks, :focus possibly

Adds support for the :active pseudo-class for hyperlinks (<a> tags
only).

Also, since it was very similar to :focus and an element having a
focused state was already implemented, I went ahead and implemented
that pseudo-class too, although I cannot come up with a working
example to validate it.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -876,6 +876,17 @@ void Document::set_focused_element(Element* element)
         m_layout_root->set_needs_display();
 }
 
+void Document::set_active_element(Element* element)
+{
+    if (m_active_element == element)
+        return;
+
+    m_active_element = element;
+
+    if (m_layout_root)
+        m_layout_root->set_needs_display();
+}
+
 void Document::set_ready_state(const String& ready_state)
 {
     m_ready_state = ready_state;

Userland/Libraries/LibWeb/DOM/Document.h

@@ -208,6 +208,10 @@ public:
 
     void set_focused_element(Element*);
 
+    const Element* active_element() const { return m_active_element; }
+
+    void set_active_element(Element*);
+
     bool created_for_appropriate_template_contents() const { return m_created_for_appropriate_template_contents; }
     void set_created_for_appropriate_template_contents(bool value) { m_created_for_appropriate_template_contents = value; }
 
@@ -323,6 +327,7 @@ private:
     bool m_editable { false };
 
     WeakPtr<Element> m_focused_element;
+    WeakPtr<Element> m_active_element;
 
     bool m_created_for_appropriate_template_contents { false };
     RefPtr<Document> m_associated_inert_template_document;

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -327,6 +327,11 @@ bool Element::is_focused() const
     return document().focused_element() == this;
 }
 
+bool Element::is_active() const
+{
+    return document().active_element() == this;
+}
+
 NonnullRefPtr<HTMLCollection> Element::get_elements_by_tag_name(FlyString const& tag_name)
 {
     // FIXME: Support "*" for tag_name

Userland/Libraries/LibWeb/DOM/Element.h

@@ -86,6 +86,8 @@ public:
     bool is_focused() const;
     virtual bool is_focusable() const { return false; }
 
+    bool is_active() const;
+
     NonnullRefPtr<HTMLCollection> get_elements_by_tag_name(FlyString const&);
     NonnullRefPtr<HTMLCollection> get_elements_by_class_name(FlyString const&);