HTML is a document markup language, not a programming language. There's no reason the site should not work in the case of markup not meeting strict standards, certainly not for the errors listed.

Also, it's not even really 66 errors - it's mostly the same three errors repeated over and over (no alt tag for images, adding a div inside a center tag and something to do with the end tag for table row.)

Although, there is also no reason for HN not to conform to standards.

Would you enjoy HN more if you couldn't browse it at all than if you browsed it with errors silently handled or ignored?

Try running it on real sites instead of simplistic ones like HN; you'll find much, much worse.