fix(domparser): preserve XML content types

Extract an independently mergeable topic from wpt-misc-fix at
a70a96f9d1.

Append only the XML content-type regression to the baseline test module.
Use empty XML to exercise the existing parser-error-document path; the original undeclared-prefix input requires a separate namespace parser fix. Content-type and error-document assertions are retained.

Source commits:
- c8eed633ac

Validation:
- cargo fmt --all
- cargo clippy --workspace --all-targets --all-features -- -D warnings
- cargo nextest run --no-fail-fast

moli-renderer-v8/src/dom_parser.rs

@@ -188,7 +188,13 @@ pub(super) fn parse_detached_document_from_string<'s>(
     } else {
         materialize_xml_parser_error_document(parsed)
     };
-    build_detached_document(scope, parsed, DetachedDocumentKind::Xml, false)
+    build_detached_document_with_content_type(
+        scope,
+        parsed,
+        DetachedDocumentKind::Xml,
+        false,
+        Some(mime),
+    )
 }
 
 fn materialize_xml_parser_error_document(parsed: NativeDom) -> NativeDom {
@@ -398,11 +404,33 @@ fn build_detached_document<'s>(
     kind: DetachedDocumentKind,
     expose_declarative_shadow_roots: bool,
 ) -> Option<v8::Local<'s, v8::Object>> {
-    build_detached_document_from_dom_host(
+    build_detached_document_with_content_type(
         scope,
-        DomHost::from_dom(parsed),
+        parsed,
         kind,
         expose_declarative_shadow_roots,
+        None,
+    )
+}
+
+fn build_detached_document_with_content_type<'s>(
+    scope: &mut v8::PinScope<'s, '_>,
+    parsed: NativeDom,
+    kind: DetachedDocumentKind,
+    expose_declarative_shadow_roots: bool,
+    content_type: Option<&str>,
+) -> Option<v8::Local<'s, v8::Object>> {
+    let _ = expose_declarative_shadow_roots;
+    let kind = match kind {
+        DetachedDocumentKind::Html => "html",
+        DetachedDocumentKind::Xml => "xml",
+    };
+    build_detached_document_object_from_dom_host_with_content_type(
+        scope,
+        kind,
+        DomHost::from_dom(parsed),
+        content_type,
+        None,
     )
 }
 

moli-renderer-v8/src/script_vm/tests/dom_elements/detached.rs

@@ -8699,3 +8699,37 @@ fn detached_domparser_adopted_nodes_follow_live_tree_for_children_text_and_mutat
         r#"{"beforeRemoval":{"firstChildIsHeld":true,"childParentIsForeignRoot":true,"childNodesLength":2,"lastChildType":3,"textContent":"xy","containsHeldChild":true},"afterRemoval":{"removedIsHeld":true,"removedParentIsNull":true,"childNodesLength":1,"firstChildType":3,"textContent":"y","liveBodyText":"y"}}"#
     );
 }
+
+#[test]
+fn domparser_xml_preserves_requested_content_type_for_success_and_error_documents() {
+    let mut vm = new_storage_test_vm("https://domparser-xml-content-type.test/");
+
+    let result = vm
+        .eval(
+            r#"
+(() => {
+  const parser = new DOMParser();
+  return JSON.stringify([
+    "text/xml",
+    "application/xml",
+    "application/xhtml+xml",
+    "image/svg+xml"
+  ].map(contentType => {
+    const valid = parser.parseFromString("<root/>", contentType);
+    const invalid = parser.parseFromString("", contentType);
+    return [
+      valid.contentType,
+      invalid.contentType,
+      invalid.documentElement.localName
+    ];
+  }));
+})()
+"#,
+        )
+        .expect("DOMParser XML content type probe should evaluate");
+
+    assert_eq!(
+        result,
+        r#"[["text/xml","text/xml","html"],["application/xml","application/xml","html"],["application/xhtml+xml","application/xhtml+xml","html"],["image/svg+xml","image/svg+xml","html"]]"#
+    );
+}