Any web company should have a policy of making its pages validate to the extent that is possible. And we do.

In the real world -- when you have to do Stupid Web Tricks to make things work in IE6, or you have advertisers who can insert random awful FONT tags -- a fully validating page is an impossible dream, especially when (for the front page, for instance) that page is different for every single user in scores of countries and dozens of languages.

So we get as close as is practical, and rely on the wonderful resilience of the web to take care of the 3% of edge cases.