Refactor *-specificity tests and documentation to align with standard CSS

[romainmenke]

Which issue, if any, is this issue related to?

Closes #7495

Is there anything in the PR that needs further explanation?

As explained in the linked issue :global() and :local() are non-standard functional pseudo classes that have these properties:

• have no specificity of their own
• do not support selector lists, only a single selector at a time (i.e. :local(a), :local(b) and not :local(a, b))

But they were used in tests to verify the ignoreSelectors feature.
This feature acts like a hole punch. Only the ignored node is skipped when calculating specificity. The child nodes are still part of the equation.

Given that :global and :local have no specificity of their own and do not allow lists, it isn't ideal that we are using these to test ignoreSelectors. All of the complexity for ignoreSelectors is in calculating the specificity of lists of child selectors.

This change replaces :global and :local with :is and :has because these do support lists of selectors (e.g. :is(a, b)).

It also adds tests with :host because :host does have its own specificity.

This gives us meaningful coverage for ignoreSelectors in a way that aligns with standard CSS.

---

The goal of this change is to have coverage in a way that makes it easier to change the implementation of this rule.

The goal is not to make breaking changes that are noticeable to end users, only to make sure that we are testing the right things in the right way.

---

Side note:

Calculating correct specificity for :global, :local or any other selector is purely an upstream concern in @csstools/selector-specificity. So I think it is fine if we don't have explicit coverage for these (non-standard) selectors.

We need coverage for the rule and for ignoreSelectors here, but not for the specificity of any individual selector.

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[Mouvedia]

Can you update

stylelint/lib/rules/no-descending-specificity/__tests__/index.mjs

Line 84 in 22f2884

code: ':global(.buildResultsSummaryTable) span {} :global(.buildResultsSummaryTable) span:nth-child(2) {}',

so that it uses either :host(), :is() or :has() instead?

[ybiquitous]

Thanks, I like this change. Can you fix the test failures?

[ybiquitous]

Thank you, LGTM 👍🏼

[romainmenke]

Thank you for the reviews, in particular from @Mouvedia 🙇