I also think it's pretty likely that if a website has fully valid HTML, that website makes little to no money. Because if it actually was a revenue-generating venture it's extremely unlikely the person or people behind it would be worrying about what tags have which attributes and which tags are self-closing and which aren't when it has zero impact on revenue.

Oh yeah, it's a labor of love. But it might be the corporate website of a business that makes a lot of money, not through the website though.