LibWeb: Add getElementsByTagNameNS and add support for * in non-NS

This also moves getElementsByTagName to ParentNode to remove the code
duplication between Document and Element. This additionally fixes a bug
where getElementsByTagName did not check if the element was a
descendant, meaning it would also include the context element if the
condition matched.

Userland/Libraries/LibWeb/DOM/ParentNode.cpp

@@ -9,6 +9,7 @@
 #include <LibWeb/DOM/HTMLCollection.h>
 #include <LibWeb/DOM/ParentNode.h>
 #include <LibWeb/Dump.h>
+#include <LibWeb/Namespace.h>
 
 namespace Web::DOM {
 
@@ -87,4 +88,68 @@ NonnullRefPtr<HTMLCollection> ParentNode::children()
     });
 }
 
+// https://dom.spec.whatwg.org/#concept-getelementsbytagname
+// NOTE: This method is only exposed on Document and Element, but is in ParentNode to prevent code duplication.
+NonnullRefPtr<HTMLCollection> ParentNode::get_elements_by_tag_name(FlyString const& qualified_name)
+{
+    // 1. If qualifiedName is "*" (U+002A), return a HTMLCollection rooted at root, whose filter matches only descendant elements.
+    if (qualified_name == "*") {
+        return HTMLCollection::create(*this, [this](Element const& element) {
+            return element.is_descendant_of(*this);
+        });
+    }
+
+    // FIXME: 2. Otherwise, if root’s node document is an HTML document, return a HTMLCollection rooted at root, whose filter matches the following descendant elements:
+    //           (It is currently always a HTML document)
+    return HTMLCollection::create(*this, [this, qualified_name](Element const& element) {
+        if (!element.is_descendant_of(*this))
+            return false;
+
+        // - Whose namespace is the HTML namespace and whose qualified name is qualifiedName, in ASCII lowercase.
+        if (element.namespace_() == Namespace::HTML)
+            return element.qualified_name().to_lowercase() == qualified_name.to_lowercase();
+
+        // - Whose namespace is not the HTML namespace and whose qualified name is qualifiedName.
+        return element.qualified_name() == qualified_name;
+    });
+
+    // FIXME: 3. Otherwise, return a HTMLCollection rooted at root, whose filter matches descendant elements whose qualified name is qualifiedName.
+}
+
+// https://dom.spec.whatwg.org/#concept-getelementsbytagnamens
+// NOTE: This method is only exposed on Document and Element, but is in ParentNode to prevent code duplication.
+NonnullRefPtr<HTMLCollection> ParentNode::get_elements_by_tag_name_ns(FlyString const& nullable_namespace, FlyString const& local_name)
+{
+    // 1. If namespace is the empty string, set it to null.
+    String namespace_ = nullable_namespace;
+    if (namespace_.is_empty())
+        namespace_ = {};
+
+    // 2. If both namespace and localName are "*" (U+002A), return a HTMLCollection rooted at root, whose filter matches descendant elements.
+    if (namespace_ == "*" && local_name == "*") {
+        return HTMLCollection::create(*this, [this](Element const& element) {
+            return element.is_descendant_of(*this);
+        });
+    }
+
+    // 3. Otherwise, if namespace is "*" (U+002A), return a HTMLCollection rooted at root, whose filter matches descendant elements whose local name is localName.
+    if (namespace_ == "*") {
+        return HTMLCollection::create(*this, [this, local_name](Element const& element) {
+            return element.is_descendant_of(*this) && element.local_name() == local_name;
+        });
+    }
+
+    // 4. Otherwise, if localName is "*" (U+002A), return a HTMLCollection rooted at root, whose filter matches descendant elements whose namespace is namespace.
+    if (local_name == "*") {
+        return HTMLCollection::create(*this, [this, namespace_](Element const& element) {
+            return element.is_descendant_of(*this) && element.namespace_() == namespace_;
+        });
+    }
+
+    // 5. Otherwise, return a HTMLCollection rooted at root, whose filter matches descendant elements whose namespace is namespace and local name is localName.
+    return HTMLCollection::create(*this, [this, namespace_, local_name](Element const& element) {
+        return element.is_descendant_of(*this) && element.namespace_() == namespace_ && element.local_name() == local_name;
+    });
+}
+
 }