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.cpp

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/HTML/DOMParser.h>
+#include <LibWeb/HTML/Parser/HTMLDocumentParser.h>
+
+namespace Web::HTML {
+
+DOMParser::DOMParser()
+{
+}
+
+DOMParser::~DOMParser()
+{
+}
+
+// https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#dom-domparser-parsefromstring
+NonnullRefPtr<DOM::Document> DOMParser::parse_from_string(String const& string, String const& type)
+{
+    // FIXME: Pass in this's relevant global object's associated Document's URL.
+    auto document = DOM::Document::create();
+    document->set_content_type(type);
+
+    // NOTE: This isn't a case insensitive match since the DOMParserSupportedType enum enforces an all lowercase type.
+    if (type == "text/html") {
+        // FIXME: Set document's type to "html".
+        HTMLDocumentParser parser(document, string, "UTF-8");
+        // FIXME: This is to match the default URL. Instead, pass in this's relevant global object's associated Document's URL.
+        parser.run("about:blank");
+    } else {
+        dbgln("DOMParser::parse_from_string: Unimplemented parser for type: {}", type);
+        TODO();
+    }
+
+    return document;
+}
+
+}