Add CSS Validator

[windymilla]

Is this necessary, or is it now covered by the Nu HTML checker?

[tangledhelix]

I didn't test exhaustively, but I ran a few HTML files with intentionally-broken CSS through Nu, and it caught them. But Nu doesn't offer all the same options, as far as I can tell - you can't validate against CSS 2.1, for example, which is what we are supposed to do unless we know CSS3 will be okay (e.g., drop-caps). So it could be using CSS separately is the proper choice for now.

[tangledhelix]

Is this issue about bundling the Nu validator or a different validator with the app?

If so, IMO we should either not do that, and instead point users to the online validator, or at minimum move this to future feature New feature or request, but not core instead of core feature Required for basic PPing .