Steps to replicate the issue (include links if applicable):
As a part of trying to make Wikimedia Commons compatible with upcoming night mode I faced this issue,
Try to do the same as https://commons.wikimedia.org/?diff=889292136 in https://commons.wikimedia.org/wiki/Template:Cc-by-sa-layout/styles.css#L-11 and use border: var(--border-subtle, 2px solid #c8ccd1); instead of currently border: 2px solid #e0e0e0;
What happens?:
It fails to save giving this error instead of save,
What should have happened instead?:
It should allow use of var in border or at least should be consistent with inline style sanitization rules.
Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):
It apparently happened here https://github.com/wikimedia/css-sanitizer/commit/34b37a6a1c9a2e8f8f2d10023f3e6d51051e1a07
It's related to T361934