Does W3C Valid HTML / CSS Help Rank?
Above – a Google video confirming this advice I first shared in 2008.
Does Google rank a page higher because of valid code? The short answer is no, even though I tested it on a small-scale test with different results.
Google doesn’t care if your page is valid HTML and valid CSS. This is clear – check any top ten results in Google and you will probably see that most contain invalid HTML or CSS. I love creating accessible websites but they are a bit of a pain to manage when you have multiple authors or developers on a site.
If your site is so badly designed with a lot of invalid code even Google and browsers cannot read it, then you have a problem.
Where possible, if commissioning a new website, demand, at least, minimum web accessibility compliance on a site (there are three levels of priority to meet), and aim for valid HTML and CSS. Actually, this is the law in some countries although you would not know it, and be prepared to put a bit of work in to keep your rating.
Valid HTML and CSS are a pillar of best practice website optimisation, not strictly a part of professional search engine optimisation. It is one form of optimisation Google will not penalise you for.
Addition – I usually still aim to follow W3C recommendations that help deliver a better user experience;
Hypertext links. Use text that makes sense when read out of context. W3C Top Ten Accessibility Tips