LibWeb: Implement ParentNode.children

Required by Web Platform Tests for the innerHTML/outerHTML tests.

Userland/Libraries/LibWeb/DOM/Document.idl

@@ -64,6 +64,8 @@ interface Document : Node {
     Element? querySelector(DOMString selectors);
     ArrayFromVector querySelectorAll(DOMString selectors);
 
+    [SameObject] readonly attribute HTMLCollection children;
+
     // FIXME: These should all come from a GlobalEventHandlers mixin
     attribute EventHandler onabort;
     attribute EventHandler onauxclick;

Userland/Libraries/LibWeb/DOM/DocumentFragment.idl

@@ -12,4 +12,6 @@ interface DocumentFragment : Node {
     Element? querySelector(DOMString selectors);
     ArrayFromVector querySelectorAll(DOMString selectors);
 
+    [SameObject] readonly attribute HTMLCollection children;
+
 };

Userland/Libraries/LibWeb/DOM/Element.idl

@@ -31,4 +31,6 @@ interface Element : Node {
 
     Element? querySelector(DOMString selectors);
     ArrayFromVector querySelectorAll(DOMString selectors);
+
+    [SameObject] readonly attribute HTMLCollection children;
 };

Userland/Libraries/LibWeb/DOM/ParentNode.cpp

@@ -6,6 +6,7 @@
 
 #include <LibWeb/CSS/Parser/Parser.h>
 #include <LibWeb/CSS/SelectorEngine.h>
+#include <LibWeb/DOM/HTMLCollection.h>
 #include <LibWeb/DOM/ParentNode.h>
 #include <LibWeb/Dump.h>
 
@@ -75,4 +76,15 @@ u32 ParentNode::child_element_count() const
     return count;
 }
 
+// https://dom.spec.whatwg.org/#dom-parentnode-children
+NonnullRefPtr<HTMLCollection> ParentNode::children()
+{
+    // The children getter steps are to return an HTMLCollection collection rooted at this matching only element children.
+    // FIXME: This should return the same HTMLCollection object every time,
+    //        but that would cause a reference cycle since HTMLCollection refs the root.
+    return HTMLCollection::create(*this, [this](Element const& element) {
+        return is_parent_of(element);
+    });
+}
+
 }

Userland/Libraries/LibWeb/DOM/ParentNode.h

@@ -25,6 +25,8 @@ public:
     ExceptionOr<RefPtr<Element>> query_selector(StringView);
     ExceptionOr<NonnullRefPtrVector<Element>> query_selector_all(StringView);
 
+    NonnullRefPtr<HTMLCollection> children();
+
 protected:
     ParentNode(Document& document, NodeType type)
         : Node(document, type)

Userland/Libraries/LibWeb/TreeNode.h

@@ -398,6 +398,15 @@ public:
         return nullptr;
     }
 
+    bool is_parent_of(T const& other) const
+    {
+        for (auto* child = first_child(); child; child = child->next_sibling()) {
+            if (&other == child)
+                return true;
+        }
+        return false;
+    }
+
     ~TreeNode()
     {
         VERIFY(!m_parent);