LibWeb: Amend Element interface to make it compatible with TrustedTypes

Author: https://github.com/tete17
Commit: db41ea8117
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/6057
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/Lubrsi ✅

Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -573,7 +573,7 @@ WebIDL::ExceptionOr<void> XMLHttpRequest::send(Optional<DocumentOrXMLHttpRequest
         // 2. If body is a Document, then set this’s request body to body, serialized, converted, and UTF-8 encoded.
         if (body->has<GC::Root<DOM::Document>>()) {
             auto string_serialized_document = TRY(body->get<GC::Root<DOM::Document>>().cell()->serialize_fragment(HTML::RequireWellFormed::No));
-            m_request_body = Fetch::Infrastructure::byte_sequence_as_body(realm, string_serialized_document.bytes());
+            m_request_body = Fetch::Infrastructure::byte_sequence_as_body(realm, string_serialized_document.to_utf8().bytes());
         }
         // 3. Otherwise:
         else {