I... don't think this \9 is intentional! If I remember correctly, any attribute value would mess up IE < 9.
At this time, I was favoring using conditional comments to deliver tweaked simplified CSSs for the IEs. They were not only buggy, they were slow, so for a while it made some sense.
There were two things, \0 (escaped null) and \9 (which I think was could be any other escape sequence, but that 9 was customarily used). I don't remember the difference in browsers they'd target, the information will still be out there somewhere, I'm sure, though you'll have to sift through dead links.
At this time, I was favoring using conditional comments to deliver tweaked simplified CSSs for the IEs. They were not only buggy, they were slow, so for a while it made some sense.