checkLess.php does not validate skin styles. This is a general issue, but is especially problematic for mediawiki.ui and mediawiki.ui.button which package all their styles this way (using 'default').
checkLess.php uses ResourceLoaderFileModule's public getAllStyleFiles method, which only uses $this->styles .
Version: 1.23.0
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=54665