Bug 235400

Summary: Implement at-rule function for use with @supports
Product: WebKit Reporter: Bramus <bramus>
Component: CSSAssignee: Nobody <webkit-unassigned>
Status: NEW ---    
Severity: Normal CC: koivisto, webkit-bug-importer
Priority: P2 Keywords: InRadar
Version: WebKit Nightly Build   
Hardware: Unspecified   
OS: Unspecified   

Description Bramus 2022-01-20 05:31:03 PST
As per CSS WG Decision [1]: Implement at-rule function for use with @supports

Syntax:

```css
/* Just the at-rule name, which returns whether this is a recognized at-rule name at all. */
@supports(at-rule(@foo)) {
  /* @foo is supported */
}
```

```css
/* An at-rule name accompanied by a descriptor declaration, which returns whether the at-rule is recognized, and the given descriptor successfully parses as part of that at-rule.*/
@supports(at-rule(@foo; desc: val)) {
  /* @foo with the descriptor `desc` and value `val` is supported */
}
```

Note that there's a follow-up issue [2] to extend at-rule() with a third syntax which allows passing in an entire at-rule into at-rule().

```css
/* An entire at-rule, which returns whether or not the at-rule as a whole, when parsed as the first and only content in a fresh stylesheet, is valid or dropped. If the at-rule is valid but drops some of its contents as invalid, such as an unknown descriptor, this will still return true. */
@supports(at-rule(@foo bar {baz: qux})) {
  /* The entire at-rule is supported */
}
```

It also supports 
At the moment, this third syntax is still a WIP.

^[1] https://github.com/w3c/csswg-drafts/issues/2463#issuecomment-1016720310
^[2] https://github.com/w3c/csswg-drafts/issues/6966
Comment 1 Radar WebKit Bug Importer 2022-01-21 08:24:55 PST
<rdar://problem/87884897>