The last time I tried to get CSS linting happening in core, I was told that Moodle does not have any CSS coding guidelines.
The ones you might find on the wiki are apparently just somebodies suggestions, and the Moodle core dev team have never committed to following them.
This seems inconsistent to me, since they have solid guidelines for PHP (that even go so far as to complain about the punctuation in PHP comments!) and they've just adopted a very in-depth JS linting strategy, with plans to expand it further.
Meanwhile on the CSS side the automation stops at running csslint with this very minimal config:
--errors=errors,duplicate-properties
--warnings=known-properties,display-property-grouping,empty-rules,important
csslint itself is a bit old and outdated, and there's more modern options, I like:
https://github.com/stylelint/stylelint