Originally Posted by Valloric
True, but both Sigil and FlightCrew wouldn't be hurt by preparations for EPUB3. Yes, that spec is still a work in progress, but it won't be for long. I'd hate to start adding checks that I'll have to remove in a few months.
If I were doing a validator, I would issue a warning (not an error) at the use of any CSS property or value not supported in the spec. It shouldn't be too hard to later add an option to validate against other versions of the spec.
In Sigil, it would be nice if unsupported properties could be disabled, so that they don't show in the book view.