Use an HTML Validator on SO
Does SO family care about valid html markup and semantic web? I've noticed that tables are used in most places where non-tabular data is being displayed.
Apparently we can safely say that valid HTML markup isn't their top priority.
Then again, I for one would not blame the developers for focusing their efforts more on the functionality and user experience of the sites...
Edit: even if strict standards compliance doesn't seem to be of utmost importance, they clearly do care about accessibility and interoperability, as the SO family sites work reasonably well with most (or all) of commonly used browsers.