The conspicuous lack of a "Server:" header inclines me to believe that that's probably not the case (most web servers set one indicating the server software and version). Here are the headers that HN sends out from an old post (20 days ago):
HTTP/1.1 200 OK
Content-Type: text/html; charset=utf-8
By ensuring that your pages are valid, you make it ever so much more likely that you will not have to scramble around wasting time at a most inopportune time when the new version of a browser comes out which handles your non-standards compliant tag soup differently than the current version of the browser.
So, do you want to pay the price upfront when you can plan for it or afterwards when the fix must be done immediately because customers are complaining?
Why don't you bother doing your real work right the first time? As long as there's a well defined spec, you might as well follow it instead of being creative and original when it comes to implementing standards.