This plugin checks if the CSS you're using is supported by the browsers you're targeting. It uses doiuse to detect browser support. Doiuse itself checks your code against the caniuse database and uses browserslist to get the list of browsers you want to support. Doiuse and this plugin are only compatible with standard css syntax, so syntaxes like scss, less and others aren't supported. This is a good rule to use with "warning"-level severity, because its primary purpose is to warn you that you are using features not all browsers fully support and therefore ought to provide fallbacks. But the warning will continue even if you have a fallback in place (it doesn't know); so you probably do not want this rule to break your build. Instead, consider it a friendly reminder to double-check certain spots for fallbacks.