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
Andrey Mnatsakanov
53295faa59