I think it's a mistake to conflate being correct (technical property) with being right (moral judgement). Adhering to a standard is quite optional, and most HTML ever written does not. That's just the way it is. This isn't wrong, but it is incorrect.
The question is, is adhering to the HTML Standard (in other words, getting zero errors in the HTML validator) beneficial to websites? For example, does it make the job of web developers easier? Are there fewer user-reported issues in browsers?