LibWeb: Make the innerHTML setter spec compliant

This adds innerHTML to ShadowRoot in the process.

Userland/Libraries/LibWeb/DOMParsing/Algorithms.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/DocumentFragment.h>
+#include <LibWeb/DOM/ExceptionOr.h>
+#include <LibWeb/HTML/Parser/HTMLDocumentParser.h>
+
+namespace Web::DOMParsing {
+
+// https://w3c.github.io/DOM-Parsing/#dfn-fragment-parsing-algorithm
+static DOM::ExceptionOr<NonnullRefPtr<DOM::DocumentFragment>> parse_fragment(String const& markup, DOM::Element& context_element)
+{
+    // FIXME: Handle XML documents.
+
+    auto new_children = HTML::HTMLDocumentParser::parse_html_fragment(context_element, markup);
+    auto fragment = make_ref_counted<DOM::DocumentFragment>(context_element.document());
+
+    for (auto& child : new_children) {
+        // I don't know if this can throw here, but let's be safe.
+        auto result = fragment->append_child(child);
+        if (result.is_exception())
+            return result.exception();
+    }
+
+    return fragment;
+}
+
+}