WPT xhr/responsexml-basic, responsexml-media-type, and responsexml-non-well-formed reveal cases where an XMLHttpRequest shouldn't be producing an XML document but is:

• an empty body
• a body that's syntactically XML, but with a Content-Type header that says it's something else
• lightly malformed XML

By WPT expectations and other browser behavior, these shouldn't parse into an XML document when they're the response to an XHR.