They do care about markup, but they care more about reaching the most audiences with the best experience. To do that they have to sacrifice valid W3C markup for

They do care about markup, but they care more about reaching the most audiences with the best experience. To do that they have to sacrifice valid W3C markup for markup that will display best in most browsers (and sometimes with browser-specific conditions). Another problem is the lack of markup dedicated to layout which sometimes breaks validity.