[css-anchor-1] position-fallback fails to cascade

[fantasai]

Reading through the issues, apparently rules in the @try blocks apply “at used value time”, which means they don't fit into the cascade at all—even !important rules won't apply.

This is problematic to begin with, and it becomes more problematic the more properties you allow in @try.

[xiaochengh]

In general, making @try rules cascadable (which I assume is equivant to applying them at computed value time) is very hard because we need to run layout to decide which fallback style to apply. Maybe we can postpone this to level 2.

But preventing them from overriding !important style should be implementable, and I agree that should be probably be done now for sanity.

[xiaochengh]

Btw is this a duplicate of #9149?

[tabatkins]

Yes, this is a #9149 dupe. (If I'm wrong, feel free to shout, @fantasai )