Maybe I’m looking at the wrong one, but is it possible that CSSLint is depreciated, and won’t be upgraded anymore? I’m noticing increasing support requests about various properties not recognized. Is it time to go with something more recent? 
Thanks!
It does look like the big css linter now is https://stylelint.io/ however I’m not sure that’s for the browser and it does look like csslint is sorta abandoned. It looks like CodeMirror switches away from csslint in 6.0, but wordpress is using 5.29 so it might take some time waiting on that. I’ll have to look more into this thanks for bringing it up!