LibWeb: Add DOMParser

This allows you to invoke the HTML document parser and retrieve a
document as though it was loaded as a web page, minus any scripting
ability.

This does not currently support XML parsing.

This is used by YouTube (or more accurately, Web Components Polyfills)
to polyfill templates.

Userland/Libraries/LibWeb/HTML/DOMParser.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/RefCounted.h>
+#include <AK/Weakable.h>
+#include <LibWeb/Bindings/Wrappable.h>
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/ExceptionOr.h>
+#include <LibWeb/Forward.h>
+
+namespace Web::HTML {
+
+// https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#domparser
+class DOMParser final
+    : public RefCounted<DOMParser>
+    , public Weakable<DOMParser>
+    , public Bindings::Wrappable {
+public:
+    using WrapperType = Bindings::DOMParserWrapper;
+
+    static DOM::ExceptionOr<NonnullRefPtr<DOMParser>> create_with_global_object(Bindings::WindowObject&)
+    {
+        return adopt_ref(*new DOMParser());
+    }
+
+    virtual ~DOMParser() override;
+
+    NonnullRefPtr<DOM::Document> parse_from_string(String const&, String const&);
+
+private:
+    DOMParser();
+};
+
+}