If you have a CSS document that contains a a @counter-style rule, CodeEditor marks it as an error.
Granted, most browsers don't support it currently (really, only Firefox does) but it is a valid CSS3 rule so I think CodeEditor should not consider it an error.
@counter-style is standard CSS. It is unsupported in WebKit (Chrome) at the moment, but it does work in Gecko (Firefox). It is useful for some languages, which need numbering style for ordered list that aren't support in predefined list-style-type values. See https://w3c.github.io/predefined-counter-styles/ for more info.
If I understand correctly, this is probably an upstream issue in Ace editor and CSSLint. I tried reporting it there: https://github.com/CSSLint/csslint/issues/770 .
And of course, it would be nice if @counter-style was supported in Chrome some day, but that's way too far from this issue :)