You can always use the validator part of
CSE HTML Validator to check the conversion, but not to actually do the conversion.
One option might be to check the document using
CSE HTML Validator's support for polyglot. Such documents can be served as XML or plain HTML5. More information is here:
http://www.htmlvalidator.com/htmlval/v1 ... l_node.htmAnother option is to try the Pretty Print & Fix (HTML Tidy) tool and force the output to 'html' and the doctype to 'html5'. Of course you'll need to check that the converted documents still work as you want and that they weren't changed in a way that breaks something.
I hope this helps.