When @counter-style is used on a CSS page, CodeEditor shows a CSS error on that line. It allows publishing the page, but it does show a warning.
@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 .
Perhaps it can be nice to patch it locally somehow, although I'm not sure it's worth the effort or even possible.