The parser will produce warnings for "minor errors" (e.g. regexps matching the empy string), but the filter can still be saved. As seen in T287436, it might be helpful to show an "Are you sure?" popup in that situation, if JS is enabled. If JS is not enabled we could show a warning in the same place where we show errors, and allow saving on the second attempt, but since this is more complicated and the no-JS case is less common, a JS popup would do for now.