[xhr2] Redirect during send question

Regarding the http redirect security violation steps, the spec (
http://dev.w3.org/2006/webapi/XMLHttpRequest/) says "If async is set to
false raise a NETWORK_ERR exception and terminate the overall algorithm."
I tried out IE7, Firefox 3, and WebKit nightlies and none of them seem to
throw an exception in this case.

Won't implementing the spec introduce a significant incompatibility?  Or did
I miss something?

Thanks,
Dave

PS I'm asking because I doing a change in WebKit in this area.