Clearly describe the bug

::-moz-focus-inner is treated as a pseudo-class, not a pseudo-element, and so can cause false positives in no-descending-specificity.

Which rule, if any, is the bug related to?

no-descending-specificity

What code is needed to reproduce the bug?

a::-moz-focus-inner {}
a {}

What stylelint configuration is needed to reproduce the bug?

e.g.

{
  "rules": {
    "no-descending-specificity": true
  }
}

Which version of stylelint are you using?

v13.7.2

How are you running stylelint: CLI, PostCSS plugin, Node.js API?

Originally encountered running it with SublimeLinter, but reproducible in the Stylelint demo page

Does the bug relate to non-standard syntax (e.g. SCSS, Less etc.)?

No

What did you expect to happen?

No errors to be flagged.

What actually happened (e.g. what warnings or errors did you get)?

The following error was flagged:

2:1 error Expected selector "a" to come before selector "a::-moz-focus-inner" (no-descending-specificity)

Fixing it

This could simply be fixed by adding -moz-focus-inner to the list of known pseudo-elements in keywordSets.js, but relying on that list is obviously not the greatest—-moz-focus-inner was actually the original example given for this bug way back in #1811, and the fix still missed it.

If we instead made the selector parsing assume any selector with a double colon is a pseudo-element, not a pseudo-class, that could be a more general fix, and would match the spec:

This :: notation is introduced by the current document in order to establish a discrimination between pseudo-classes and pseudo-elements.

Of course, the confusion between e.g. :after and ::after is well-known, and the former is in (limited, but spec-compliant) use, so we would still need a keyword list, but this would solve all syntactically valid cases going forward, and existing logic would probably handle any common invalid ones gracefully.