I do not assume, that this document is anyhow compliant to the html5 or any other html standard.
But you are assuming that it isn't compliant. You could check by reading the standard.
See, for example, the html element:
Tag omission in text/html:
An html element's start tag can be omitted if the first thing inside the html element is not a comment.
An html element's end tag can be omitted if the html element is not immediately followed by a comment.
Is there a more strict validator out there, which reports every tiny deviation from standard html?
While tool recommendations are off-topic for this site, there are no deviations from standard html in the code you've presented.
The Nu validator is very good at what it does.
Answer from Quentin on Stack OverflowI do not assume, that this document is anyhow compliant to the html5 or any other html standard.
But you are assuming that it isn't compliant. You could check by reading the standard.
See, for example, the html element:
Tag omission in text/html:
An html element's start tag can be omitted if the first thing inside the html element is not a comment.
An html element's end tag can be omitted if the html element is not immediately followed by a comment.
Is there a more strict validator out there, which reports every tiny deviation from standard html?
While tool recommendations are off-topic for this site, there are no deviations from standard html in the code you've presented.
The Nu validator is very good at what it does.
Answer from Quentin on Stack OverflowVideos
HTTP Error 500 is an Internal Server Error, so that will be their fault. Try validator.nu whilst you are waiting.
W3C validator maintainer here. Tip: Next time you see that message, try validating your document directly at http://validator.w3.org/nu/ instead.
Because if you see that 500 error it means the validator is trying to reach the HTML5 backend, which runs as a separate service/process responding at localhost:8888 on the validator host.
But we actually run several validator hosts, round-robin, so if you see that 500 error it could mean that only one of the HTML5 backends on one of the hosts is temporarily down. And if you go to http://validator.w3.org/nu/ you might get lucky and hit a different host and it'll work.
On the other hand if you get a 404 from http://validator.w3.org/nu/ it most likely means you caught me in the middle of restarting/redeploying the W3C HTML5 validator backends to pull in changes I've made to the sources from https://github.com/validator/validator
But that never takes me more than a few minutes, so on your end you should never be seeing that 500 error for more than, say, 10 minutes at most. So if/when you do see it for any longer than that, please report it either by tweeting to @w3c or @sideshowbarker on twitter, or pinging me on IRC (MikeSmith on #whatwg or #w3c on irc.freenode.net).