LibWeb: Implement XMLFragmentParser

Implement XMLFragmentParser based on the specification:
https://html.spec.whatwg.org/multipage/xhtml.html

Fixes one WPT in:
domparsing/insert_adjacent_html-xhtml.xhtml

Libraries/LibWeb/XML/XMLFragmentParser.cpp

@@ -0,0 +1,83 @@
+/*
+ * Copyright (c) 2025, mikiubo <michele.uboldi@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "XMLFragmentParser.h"
+#include <LibWeb/DOM/Document.h>
+#include <LibWeb/DOM/Element.h>
+#include <LibWeb/HTML/Parser/HTMLParser.h>
+#include <LibWeb/WebIDL/DOMException.h>
+#include <LibWeb/XML/XMLDocumentBuilder.h>
+#include <LibXML/Parser/Parser.h>
+
+namespace Web {
+
+// https://html.spec.whatwg.org/multipage/xhtml.html#parsing-xhtml-fragments
+WebIDL::ExceptionOr<Vector<GC::Root<DOM::Node>>> XMLFragmentParser::parse_xml_fragment(DOM::Element& context, StringView input, HTML::HTMLParser::AllowDeclarativeShadowRoots allow_declarative_shadow_roots)
+{
+    // 1. Create a new XML parser.
+    // NB: The feed will be used to create the parser below
+    StringBuilder feed;
+
+    StringBuilder qualified_name_builder;
+    if (auto const& prefix = context.prefix(); prefix.has_value() && !prefix->is_empty()) {
+        qualified_name_builder.append(prefix.value());
+        qualified_name_builder.append(':');
+    }
+    qualified_name_builder.append(context.local_name());
+    auto const& qualified_name = qualified_name_builder.string_view();
+
+    // 2. Feed the parser just created the string corresponding to the start tag of context,
+    feed.append('<');
+    feed.append(qualified_name);
+    feed.append('>');
+
+    // FIXME
+    //  declaring all the namespace prefixes that are in scope on that element in the DOM,
+
+    // FIXME
+    //  as well as declaring the default namespace (if any) that is in scope on that element in the DOM.
+    //  A namespace prefix is in scope if the DOM lookupNamespaceURI() method on the element would return a non-null value for that prefix.
+    //  The default namespace is the namespace for which the DOM isDefaultNamespace() method on the element would return true.
+
+    // 3. Feed the parser just created the string input.
+    feed.append(input);
+
+    // 4. Feed the parser just created the string corresponding to the end tag of context.
+    feed.append("</"sv);
+    feed.append(qualified_name);
+    feed.append(">"sv);
+
+    GC::Ptr<DOM::Document> document = DOM::Document::create(context.realm());
+    document->set_document_type(DOM::Document::Type::XML);
+    if (allow_declarative_shadow_roots == HTML::HTMLParser::AllowDeclarativeShadowRoots::Yes)
+        document->set_allow_declarative_shadow_roots(true);
+
+    XML::Parser parser(feed.string_view());
+    XMLDocumentBuilder builder { *document, XMLScriptingSupport::Disabled };
+    auto result = parser.parse_with_listener(builder);
+
+    // 5. If there is an XML well-formedness or XML namespace well-formedness error, then throw a "SyntaxError" DOMException.
+    if (result.is_error()) {
+        return WebIDL::SyntaxError::create(context.realm(), Utf16String::formatted("{}", result.error()));
+    }
+
+    auto* doc_element = document->document_element();
+
+    // 6. If the document element of the resulting Document has any sibling nodes, then throw a "SyntaxError" DOMException.
+    if (doc_element->previous_sibling() || doc_element->next_sibling()) {
+        return WebIDL::SyntaxError::create(context.realm(), "Document element has sibling nodes"_utf16);
+    }
+
+    // 7. Return the resulting Document node's document element's children, in tree order.
+    Vector<GC::Root<DOM::Node>> result_nodes;
+    for (auto* child = doc_element->first_child(); child; child = child->next_sibling()) {
+        result_nodes.append(*child);
+    }
+
+    return result_nodes;
+}
+
+}