From time to time, we get questions and comments about validating your site using an HTML validator, e.g. http://validator.w3.org. For a long time, it was considered important that WordPress sites pass a vlidator The current high level thinking at WordPress about W3C validation is that it no longer is really meaningful.
It has a lot to do with HTML5. While HTML5 is now supported by all major browsers, it is still not a final, locked-in-stone standard. So what has happened at W3C is that they can’t write a validator that really works right all the time for HTML5. So you end up having to ignore certain flags on stuff like ‘pubdate’ or ‘rel=’ fields. Keeping up with all those irrelevant warnings could drive you crazy.
So we all have to accept the new fact that no one is able to really do anything anymore based on passing the W3C validator. Really. Have to give up on it. Search Engines don’t care. No one does. HTML5 put an end to that. Anything you may have read or learned about validation has changed. It is not possible to produce an HTML5 W3C validated site. One could probably could still validate an older version of HTML, but the world has moved on for now.
And this is fairly recent – certainly within the past year. There are still many tools that talk about validation, but they are no longer relevant.
But – the W3C validator is still very useful to find deprecated usage (e.g., <center>), and unbalanced HTML tags – unclosed <div>’s, for example, and is likely to remain useful for that kind of purpose for a long time to come. And the time may come that W3C is able to properly validate HTML5 – but that likely won’t be for a long time.