Using #Feature_Queries in #CSS ★ Mozilla Hacks – the Web developer blog
There’s a tool in CSS that you might not have heard of yet. It’s powerful. It’s been there for a while. And it’ll likely become one of your favorite new things about CSS.
Behold, the #@supports rule. Also known as Feature Queries.
With @supports, you can write a small test in your CSS to see whether or not a particular “feature” (CSS property or value) is supported, and apply a block of code (or not) based on the answer.