tl;dr: I think it's safe.

[This test](https://github.com/jquery/jquery/blob/master/test/unit/ajax.js#L185) probably wouldn't pass. I know it was an issue with older IEs: IE7 and below iirc, maybe even just IE6 or IE7. I clearly remember the wtf expression on my face though (silly human brain). Fact is I discovered it thanks to a unit test... now the test suite has seen some changes but I'm quite confident it would still catch this problem if it were to arise again.