This is a rubbish service that just wraps the Validator.nu HTML Parser and XML serialiser in an HTTP interface. POST some HTML to this URL, and you should get some XHTML back.

E.g.: cat whatever-file-you-want | curl --data-binary @- -H 'Content-Type: text/html;charset=utf-8' -0 http://whatever-this-site-is-called/html-to-xhtml/

The parser is in streaming mode, so it'll abort and send a U+0018 followed by an error message if you feed it HTML that's broken in certain ways.

You probably don't want to use compression when talking to this server, since its CPU power is rubbish and can't cope with such advanced technologies, and also compression seems to break the HTTP streamingness for some reason.

If you wish, you could use this handy textarea:

Write some HTML here!

Click this button here!

Your XHTML will be delivered here!