HTML Validator -HTML5

Hi
 
I wonder if you can help.  I’ve successfully installed local copies of your markup and css validators and these  are being used by students at this University to check the validity of their HTML.  I’m now trying to add the HTML5 validator (validator.nu) .
 
This is currently failing to build due to a HTTP 503 - Service Unavailable error when the build script tries to download http://www.w3.org/TR/1999/PR-xhtml-19991210/DTD/xhtml1-strict.dtd. 
 
Can you say whether this is a temporary error at your end or if the URI is incorrect please?.
 
Regards
 
Martin Sharratt
Unix Administrator
University of the West of England
Bristol
UK