Is it possible to do a CSS @supports check on a @media rule?

Question

The @support rule allows one to do a feature query on CSS properties. I am wondering whether it is possible to do a feature check on specifically @media rules?

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Answer 1

This does not seem to work. Should it work?

No; @supports only supports property declarations, not at-rules or indeed any other grammatical construct in CSS. You're not looking to check for support for the @media rule anyway; you're trying to check for support for specific media features. But @supports doesn't support that either, even though media features share the same declaration syntax with property declarations.

To that end, you don't need the @supports rule at all. To check if a browser supports a certain media feature, simply write a @media rule with a media query list containing both the media feature and its negation:

@media not all and (pointer), (pointer) {
  p { color: green; }
}

<p>If this text is green, your browser supports the <code>pointer</code> media feature.

(Note that Media Queries 4 removes the restriction of not requiring a media type from MQ3, so the negation really ought to be not (pointer), but no browser supports this yet and a media type is still required.)

Browsers that don't recognize the pointer media feature will interpret the @media rule as @media not all, not all (in spite of the not in the not all and (pointer) media query). See section 3.2 of the spec, which says

An unknown <mf-name> or <mf-value>, or disallowed <mf-value>, results in the value “unknown”. A <media-query> whose value is “unknown” must be replaced with not all.

If you need to apply CSS for when a browser does not support a media feature, these error handling rules mean you'll need to take advantage of the cascade (and if you don't know the original values in advance, you may be stuck):

p { color: red; }

@media not all and (pointer), (pointer) {
  p { color: currentcolor; }
}

<p>If this text is red, your browser does not support the <code>pointer</code> media feature.