I've noticed a lot of sites, SO included, use XHTML as their mark-up language and then fail to adhere to the spec. Just browsing the source for SO there are missing closing tags for paragraphs, invalid elements, etc.
So should tools (and developers) use the XHTML doctype if they are going to produce invalid mark up? And should browsers be more firm in their acceptance of poor mark-up?
And before anyone shouts hypocrite, my blog has one piece of invalid mark-up involving the captha (or it did the last time I checked) which involves styling the noscript tag.