LibWeb: Make the innerHTML setter spec compliant

This adds innerHTML to ShadowRoot in the process.

Userland/Libraries/LibWeb/DOMParsing/InnerHTML.h

@@ -0,0 +1,40 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/DOM/ExceptionOr.h>
+#include <LibWeb/DOM/ShadowRoot.h>
+#include <LibWeb/DOMParsing/Algorithms.h>
+#include <LibWeb/HTML/HTMLTemplateElement.h>
+
+namespace Web::DOMParsing::InnerHTML {
+
+// https://w3c.github.io/DOM-Parsing/#dom-innerhtml-innerhtml
+static DOM::ExceptionOr<void> inner_html_setter(NonnullRefPtr<DOM::Node> context_object, String const& value)
+{
+    // 1. Let context element be the context object's host if the context object is a ShadowRoot object, or the context object otherwise.
+    //    (This is handled in Element and ShadowRoot)
+    NonnullRefPtr<DOM::Element> context_element = is<DOM::ShadowRoot>(*context_object) ? *verify_cast<DOM::ShadowRoot>(*context_object).host() : verify_cast<DOM::Element>(*context_object);
+
+    // 2. Let fragment be the result of invoking the fragment parsing algorithm with the new value as markup, and with context element.
+    auto fragment_or_exception = parse_fragment(value, context_element);
+    if (fragment_or_exception.is_exception())
+        return fragment_or_exception.exception();
+    auto fragment = fragment_or_exception.release_value();
+
+    // 3. If the context object is a template element, then let context object be the template's template contents (a DocumentFragment).
+    if (is<HTML::HTMLTemplateElement>(*context_object))
+        context_object = verify_cast<HTML::HTMLTemplateElement>(*context_object).content();
+
+    // 4. Replace all with fragment within the context object.
+    context_object->replace_all(fragment);
+
+    return {};
+}
+
+}