At the time of writing this, Google had 40, Yahoo 154, Facebook 41, Twitter 88. All are validation errors on their homepages. No matter whether they define their document type as being HTML 4.01, HTML5 or XHTML 1.0, they all fail to validate and break basic rules. It's worth to mention that Google is represented in the HTML working group at W3C and they also handle the HTML5 draft editing. That's one example.
But why would they make such flagrant breakings of the web standards? They, in fact, produce incorrect code. Client code. And then make it inevitably public and available for others to see how they've done it. These are some of the most visited and busiest sites out there, and yet they see the rules of the web as something optional. Web browsers (some more than others) are often criticized for not providing a correct rendering of pages and for making it difficult for developers to build consistent applications. Very true. But do developers respect what they claim to be respected? Seems not. And this is just a (representative) sample, because there are only a few sites having valid markup. And even though an end user may not notice a simple HTML error, or may not care about it, the web standards have a clear goal of making things overall better for end users, while being transparent to them. Markup is the support framework of any content. And in the long run, a healthy markup will sustain better content. Better = correct, accessible and meaningful.