Why, all of them, of course!
The devices should be maintained with soft/firm-ware updates with the latest versions. They also need to be backwards compatible to support all the older books. The opf states what version (2/3) the epub is, so it shouldn't be too hard for a reader to apply the correct function...if there are any differences.
Pseudo elements, in particular, have been around for a very long time. A device that doesn't support that css is, imho, broken...
That standard of treating unknowns as a null operation is to prevent books from crashing. If your device is old (no longer supported and/or company is out of business), or can't do color (eink), or something, then the device would ignore that css.