This page studies the @supports rule in a few specific situations.
So far mostly tested on desktop. Mobile tests will follow later (I hope).
The problem with this declaration is that most mobile browsers support either local or fixed, but not both. Some don’t even support one of the two. As I read the spec, they should report that they do not support the values that don’t actually do anything useful. But they declare support for both, which is wrong.
With the exception of Safari, all browsers will always claim support for any value of transition-property; even nonsense. This is obviously wrong.
More subtle is the case of display. Yes, it’s a valid property name, and thus a valid value for transition-property, but you can’t actually transition a display declaration. Should @supports still claim support, even if it does nothing useful?
So far @supports only works on declarations and values; not on selectors or media queries. Pity, but such is life.
The syntax of the examples may actually appear a future version of the spec. On the other hand, it may not.
Other stuff I tested that is all correct.