Discovered last week: @.supports (transition-property: nonsense) returns true in Chrome. But Chromium 34 and below return false.

I found a false positive in Firefox @support {appearance: none} in version 33. The browsers really need to be careful with that.

I think the issue you're seeing is that we've had to add more behaviors to -moz-appearance:none, copying other browsers.

You can't test for a specific effect that it has, only whether it's accepted, which should mean it does something (it did)

The 'appearance' property specifically is a mess; there's no spec for form controls or how 'appearance' affects them.

So we end up having to copy different behaviors from other browsers at different times. @supports won't help there.