Feature Summary:
Add an option to block the use of experimental CSS features in signatures, especially those marked by vendor prefixes.

Note that this doesn't have to be a complicated parser. A simple -(moz|webkit)- filter might work too. Support for extra abuse filters for signatures would work too.

Use Case(s):
Due to using vendor prefixes, this user has a ridiculously long signature:
Link to user's discussion
This could have, and should have been shorter, but that is covered by T366049. The problem also arises from using CSS with a vendor prefix, which might eventually be removed (the -moz- prefix is not used anymore already by Firefox).

This could become a linter error that we should eventually clean up. So more work for future Wikipedians later... (there is an onging cleanup on pl.wikipedia with some old html tags already, e.g. font).

Benefits:
Making signatures more stable will ensure discussion pages work better in the future. It will also make the code of discussion pages more readable and shorter.