Case-sensitivity of ID selectors

[Ms2ger]

https://drafts.csswg.org/selectors-4/#id-selectors

When matching against a document which is in quirks mode, IDs must be matched ASCII case-insensitively; ID selectors are otherwise case-sensitive.

[SimonSapin]

Does a document’s mode (quirks vs not) ever change?

Relevant code in matching.rs is not only matches_simple_selector but also the SelectorMap::id_hash map.