Background
'stylelint-config-wikimedia' offers both, a Modern and a Basic list of supported browsers.
In a commit a while ago, the browser set against was 'grade-a' aka 'modern'. Assuming that was done given the many forward-looking changes by devs provided and merged all over the place in MediaWiki core.
While understandable from a linting perspective, given the many warning, it's not the right approach from a CSS support and Product perspective IMO.
Goal
Given the changes in the making on bumping up browser support in T306486 and related, we should
- replace 'grade-a' with 'basic' in MediaWiki core
with stylelint-config-wikimedia is updated to new version reflecting updated browser support matrix.