Let a pseudo-class be followed by a quoted attribute selector

[jdalton]

The combinator inside the validator's pseudo-class pattern consumes the character after it, which is the '[' of a following attribute selector, so the whole selector is rejected. Fixes #175, which reaches jsdom users through @testing-library/user-event.

Detail, and how it was checked

The combinator alternative inside the validator's pseudo-class pattern is '[>+~][^>+~]', which consumes the character after the combinator. When that character is the '[' of an attribute selector, the attribute can no longer be parsed: the validator stops mid-selector and the whole selector is rejected. The top-level combinator pattern already uses a lookahead, so this makes the two agree.

"[class*='a' i]:not(:empty) + [class*='b']" is #175. It reaches jsdom users through @testing-library/user-event, which matches every stylesheet rule when checking pointer-events, so one such rule in a stylesheet breaks unrelated tests.

The error it raises names '[class*='a' i]:not(:empty)+[class*,,,b,,' as the offending selector, because emit() is passed the array of fragments the validator did match rather than the selector; String() then joins them with commas, which reads as corrupted quotes. It now names the selector.

Extracted from #167 as a standalone change: one file, applies to master on its own, and checked against a fixture to confirm it fixes what it describes and moves nothing else.

References: the spec, the browser source, and what each part was reasoned from

This patch applies to master on its own. The sixteen in this series were checked by cherry-picking them onto master one after another, in this order and in reverse, and all sixteen land without a conflict.

• Spec: https://drafts.csswg.org/selectors-4/#attribute-selectors — attribute selectors, including the case-sensitivity flag.
• Spec: https://drafts.csswg.org/selectors-4/#attribute-case — the 'i' flag this pattern has to survive.
• MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors.