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IE had these filter things, and support for them is enabled in Compass by default. They look like:
But this totally trips up the parser.
Fortunately, versions of IE that used this sort of thing are dying out. I don't know whether the work required to fix this would be small enough to justify fixing it.
Let me know if you figure out a good way of parsing these! I'm working on a CSS3 parser of my own and IE filters are causing a lot of grief.
Looks like Ruby outright cheats and makes = an operator, which just returns its operands joined by =.
They can be parsed now! progid:whatever(...) is treated as opaque, just like expression().