The Wikimedia projects are not currently passing the W3's markup validation, e.g. see:
http://validator.w3.org/check?uri=en.wikipedia.org
Currently, 4 errors are reported.
The same also applies with CSS, where there are currently 252 bugs on en.wp:
http://jigsaw.w3.org/css-validator/validator?uri=en.wikipedia.org
(and varying but similar numbers on other projects).
Version: unspecified
Severity: normal
| Status | Subtype | Assigned | Task | ||
|---|---|---|---|---|---|
| Resolved | None | T2209 [DO NOT USE] HTML validity (tracking) | |||
| Declined | Krinkle | T37914 Wikimedia projects do not pass W3's validation |
This has been discussed several times before. We don't write for validators, we write for real browsers. Real browsers support an overlapping subset of actual standards. We do what is necessary to provide a working platform to users. None of these issues raised by the validator affect actual browsers or users, and each is there for a reason without downsides.
Either way the issues from 2012 are no longer relevant and there weren't any specific issues reported here. Most things from the validator have been reported in separate issues already and were declined for good reason or resolved.
If you find any new warnings that may be cause for concern, please create specific tasks for them.