Bugzilla – Bug 17775
DOMParser Errors should be exceptions
Last modified: 2012-07-14 07:34:36 UTC
Seems like a more robust design to throw in these cases so that developers can easily spot errors (versus returning an error document).
The claim is that it shipped this way in IE9 with no reported issues.
Given that IE often hits other code paths than other browsers, I don't believe that alone is sufficient evidence that the non-IE web doesn't depend on the specced behaviour.