From someone at work (I asked for his permission to quote him):
version of IE. There were some errors in the spec.
First step to standardization, of course, was to write down the spec for the language. Netscape didn't actually have one, outside the code itself (except perhaps whatever was written on Brendan's whiteboard) ..but hey, Microsoft happened to have that cleanroom document.
They submitted their document, and through contributing a couple orders of magnitude more manpower to the process than Netscape could afford, pretty much pushed it through. Errors and all.
And voila -- some of the Netscape browser's behaviour is now in violation of the spec for the language that they themselves created.