Fix false positives for focus-visible in selector-pseudo-class-no-unknown

[mo]

Running stylelint 9.9.0 using stylelint-config-recommended on the following file:

:focus-visible {
	color: red;
}

...reports the error:

test.css
 1:1  ✖  Unexpected unknown pseudo-class selector ":focus-visible"   selector-pseudo-class-no-unknown

:focus-visible is a new pseudo class coming in CSS Selectors v4.

It's currently implemented behind a prefix in Firefox and behind a flag in Chrome, see status here:
https://caniuse.com/#search=focus-visible

Chrome implementation is tracked by this bug:
https://bugs.chromium.org/p/chromium/issues/detail?id=817199&desc=3

Some people are already using it via this polyfill:
https://github.com/WICG/focus-visible

It would be nice if stylelint did not report this pseudo class as "unknown".

[jeddy3]

@mo Thanks for the report.

Yes, this should be considered known. For those interested, it's in the Level 4 Selector Working Draft.

It should just be the case of adding it to this list, if you'd like a submit a PR.

[mo]

@jeddy3 If I also wanted to write a testcase, in which file should I put it?

[jeddy3]

Each rule has it own test folder within.

You should just need to add a :focus-visible {} item to the accept array around this line of the selector-pseudo-class-no-unknown rule.

FYI, docs on running tests.