For the most part, I try to make the argument that support for Internet Explorer 9 should be limited to answering yes to the question “is the information available and understandable to the visitor?”. This allows us to ignore many of the little quirks that makes IE9 such a mess to implement design for. However, a quirk that is hard to ignore is how declaring
background-position: calc(100% - 10px)
will crash Internet Explorer 9 relentlessly, without so much as an error in the console. Now, just like the esoteric selector limit, this one will only surface in an actual Internet Explorer 9, and not in any compatibility mode, so make sure you’re using modern.ie or something similar in your test process.
This one recently surfaced after weeks in a production environment, with Google Analytics reporting at least a few thousand visitors each month using the steaming heap of crap that is IE9. Despite that, not a single bug report came through the door. My hunch is that Google Analytics includes those with some deviant document mode forced upon them – due to bad decisions or negligence – in the statistics. Either way, January 12 can’t come soon enough*.
*I’m well aware that “dropping support” doesn’t necessarily mean anything, and that they’re still supporting IE9 on Vista-systems, but it’s the hope that gets me through these dark winter days.