fix(js): surface a parsererror for malformed XML in DOMParser

DOMParser.parseFromString always HTML-parsed, even for XML mime types, so
malformed XML never produced the <parsererror> element Chrome emits and
error-detection code (doc.querySelector('parsererror')) never triggered. obscura
has no XML parser, so full XML semantics stay out of scope; instead, for XML
mime types run a conservative well-formedness check (tag balance, single root,
terminated comment/CDATA/PI, skipping quoted attribute regions) and, on a clear
error, make the document surface a <parsererror>. The check defaults to
well-formed when ambiguous, so valid XML is never falsely flagged.

Fixes #504

crates/obscura-js/js/bootstrap.js

@@ -25,7 +25,7 @@
     '_isSpecialScheme', '_applyDocQueryEncoding', '_anchorBase',
     '_elemHrefURL', '_setElemHrefPart', '_pad', '_daysInMonth',
     '_isoWeek1Monday', '_inputParseNumber', '_inputFormatNumber',
-    '_htmlAttrName', '_convertNodes', '_parseHTMLFragment', '_elementClassFor', '_wrap', '_wrapEl',
+    '_htmlAttrName', '_convertNodes', '_parseHTMLFragment', '_xmlWellFormed', '_elementClassFor', '_wrap', '_wrapEl',
     '_resolveUrl', '_registerIframe', '_base64ToUint8Array',
     '_bodyToUint8Array', '_arrayBufferFromBytes',
     '_installWasmStreamingFallback', '_urlParseOp', '_urlSetOp',
@@ -5464,6 +5464,56 @@ if (typeof URLSearchParams === "undefined") globalThis.URLSearchParams = class U
 // input into a detached `<html>` element and wrap it so the common Document
 // API surface (body / head / documentElement / querySelector* / getElementById /
 // getElementsByTagName / getElementsByClassName / title / cloneNode) works.
+// Conservative XML well-formedness check. obscura has no XML parser, so this
+// only decides whether to surface a <parsererror> (it does not build an XML
+// tree). It flags clear structural errors — mismatched or unclosed tags,
+// multiple/no root elements, unterminated comment/CDATA/PI — and defaults to
+// "well-formed" whenever the scan is ambiguous, so valid XML is never falsely
+// flagged. Quoted attribute regions, comments, CDATA, PIs and the doctype are
+// skipped; a literal '<' in text (invalid in XML) reads as a bad tag.
+function _xmlWellFormed(src) {
+  const s = String(src);
+  const stack = [];
+  let rootsClosed = 0; // top-level elements fully closed (or self-closed)
+  let i = 0;
+  const n = s.length;
+  while (i < n) {
+    const lt = s.indexOf('<', i);
+    if (lt === -1) break;
+    i = lt;
+    if (s.startsWith('<!--', i)) { const e = s.indexOf('-->', i + 4); if (e === -1) return false; i = e + 3; continue; }
+    if (s.startsWith('<![CDATA[', i)) { const e = s.indexOf(']]>', i + 9); if (e === -1) return false; i = e + 3; continue; }
+    if (s.startsWith('<?', i)) { const e = s.indexOf('?>', i + 2); if (e === -1) return false; i = e + 2; continue; }
+    if (s.startsWith('<!', i)) { const e = s.indexOf('>', i + 2); if (e === -1) return false; i = e + 1; continue; }
+    // A start/end/self-closing tag: find its '>' while skipping quoted regions.
+    let j = i + 1, quote = null;
+    while (j < n) {
+      const c = s[j];
+      if (quote) { if (c === quote) quote = null; }
+      else if (c === '"' || c === "'") quote = c;
+      else if (c === '>') break;
+      j++;
+    }
+    if (j >= n) return false; // unterminated tag
+    const inner = s.slice(i + 1, j).trim();
+    i = j + 1;
+    if (!inner) return false;
+    if (inner[0] === '/') {
+      const name = inner.slice(1).trim().split(/\s/)[0];
+      if (stack.length === 0 || stack[stack.length - 1] !== name) return false;
+      stack.pop();
+      if (stack.length === 0) rootsClosed++;
+    } else if (inner[inner.length - 1] === '/') {
+      if (stack.length === 0) rootsClosed++;
+    } else {
+      const name = inner.split(/\s/)[0];
+      if (!name) return false;
+      stack.push(name);
+    }
+  }
+  return stack.length === 0 && rootsClosed === 1;
+}
+
 globalThis.DOMParser = class DOMParser {
   parseFromString(source, mimeType) {
     const html = String(source ?? "");
@@ -5475,6 +5525,15 @@ globalThis.DOMParser = class DOMParser {
     // children, which is what callers want.
     try { root.innerHTML = html; } catch (e) { /* leave empty on parse error */ }
 
+    // For XML mime types, surface a <parsererror> on clearly-malformed input so
+    // error-detection code (doc.querySelector('parsererror')) works, matching
+    // Chrome. obscura has no XML parser, so the tree stays HTML-parsed.
+    if (isXml && !_xmlWellFormed(html)) {
+      try {
+        root.innerHTML = '<parsererror xmlns="http://www.w3.org/1999/xhtml">This page contains the following errors:<div>error while parsing XML</div></parsererror>';
+      } catch (e) { /* ignore */ }
+    }
+
     // Helper: depth-first walk to find an element by predicate.
     const walk = (node, pred) => {
       if (!node) return null;

crates/obscura-js/src/runtime.rs

@@ -1676,6 +1676,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn dom_parser_flags_malformed_xml_with_parsererror() {
+        let mut rt = setup_runtime("<html><body></body></html>");
+        let has_err = rt
+            .evaluate("(function(){var d=new DOMParser().parseFromString('<a><b></a>','application/xml'); return d.querySelector('parsererror') ? true : false;})()")
+            .unwrap();
+        assert_eq!(has_err, serde_json::json!(true));
+    }
+
+    #[test]
+    fn dom_parser_accepts_well_formed_xml() {
+        let mut rt = setup_runtime("<html><body></body></html>");
+        let ok = rt
+            .evaluate("(function(){var d=new DOMParser().parseFromString('<root><child>x</child></root>','application/xml'); return d.querySelector('parsererror') ? 'ERR' : 'OK';})()")
+            .unwrap();
+        assert_eq!(ok, serde_json::json!("OK"));
+    }
+
+    #[test]
+    fn dom_parser_html_never_gets_parsererror() {
+        // HTML parsing is tolerant and must never synthesize a parsererror.
+        let mut rt = setup_runtime("<html><body></body></html>");
+        let ok = rt
+            .evaluate("(function(){var d=new DOMParser().parseFromString('<div><p>hi</a>','text/html'); return d.querySelector('parsererror') ? 'ERR' : 'OK';})()")
+            .unwrap();
+        assert_eq!(ok, serde_json::json!("OK"));
+    }
+
     #[test]
     fn test_document_title() {
         let mut rt = setup_runtime("<html><head><title>Test</title></head><body></body></html>");