Created by: cvrebert
The obvious, simple fix for #5494 (closed). Removes the :required selectors from the :invalid styles so that optional fields with invalid values also get styled.
The commit which originally introduced these styles (1209a375) does not explain the inclusion of the requireds, nor does the blog post to which it links (said post instead justifies the :focuses), so the exclusion of optional fields doesn't seem to have been deliberate / explicitly-intended.