feat: add support for case-insensitive attribute selectors

[iChenLei]

What:

add support for case-insensitive attribute selectors
try to fix #3027 #3670 go-gitea/gitea#15520 go-gitea/gitea#15565

Why:

CSS spec -> https://www.w3.org/TR/selectors-4/#attribute-case

How:

#3027 (comment)

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

cc @matthew-dean

[matthew-dean]

Looks great! Thanks so much! One thing I'm thinking of -- IIRC, I believe there are tests somewhere that test attribute equality. Essentially there are tests like:

.foo when ([attr=value] = [attr="value"]) { }

So we may want to add this attribute selector to the equality check as well. IMO I feel like these equality checks are an overly-specific feature of Less, but since they exist, we want to make sure they don't break or that equality isn't true when the additional cif is present on one side of the comparison (and is true when, for example, one side side has a different casing than the other).

.foo when ([attr="value" i] = [attr="value" I]) { } // should be true and output .foo {}

In other words, the flag should probably serialize to the lower-case variant. Probably? What do you think? I don't want to over-engineer / over-think this, but I thought I should at least make you aware of it.

[iChenLei]

@matthew-dean Thanks for review, will add more unit test soon. 😄

[iChenLei]

@matthew-dean Sir, looks like current not support use when to compare attrbuite selectors(without case-insensitive flag) ?

[matthew-dean]

@iChenLei Oh, no? Then never mind! Thanks for checking it out!

[iChenLei]

@matthew-dean puppeteer e2e test often fail. 😢 https://github.com/less/less.js/runs/4619921788?check_suite_focus=true