Oh wow, the @support rule is a chaos once you dive deeper.

How so? Support-wise, or in terms of its actual definition?

Support-wise. Though I made one mistake. But background-attachment: fixed | local is worth looking at in mobile browsers.

I would be VERY interested to know more. I’m starting to use it in production and plan to write about it soon.

And do you have any idea if you're supposed to be able to detect media query support by @supports?

That is a very good question, and I don’t think so; I believe it can only reference properties and/or values. (Paging @davidbaron…)