http://lists.w3.org/Archives/Public/public-webapps/2011OctDec/1326.html
If/when you do it, make sure you don't let sync XHRs use this response type
Also, we should throw away the source string when we're done to save memory. It's spec'ed that once you parse it into an object you no longer need the source string (i.e. responseText no longer returns the source string).
*** This bug has been marked as a duplicate of bug 73648 ***