I just got surprised to know that google.com is not a valid XHTML page. But yes, it is true, you can yourself check it on the W3C validator.
Here is the explanation by Matt Cutts, head of Google’s webspam team, about why the Google homepage is not a valid HTML5 (or XHTML) document.
Matt Cutts says:-
1) It is not like Google developers cannot make the page valid, but as the site is opened on numerous browsers and devices, there are lot many factors involved
2) It is not like Google boosts the sites with valid XHTML source while crawling, as there are many sites on web which are not with valid html source.
Watch the video and let us know your comments on it.