Problem with request to nu.validator

I have a web app that teaches coding.

In a couple of the lessons the learners have to validate their html and fix
errors.

The validator requests no longer work.
I get the following error:

Are we no longer allowed to use this api for client side requests... or is
this an oversight?

XMLHttpRequest cannot load http://html5.validator.nu/. No
'Access-Control-Allow-Origin' header is present on the requested resource.
Origin 'http://www.codeavengers.com' is therefore not allowed access.

-- 
Mike Walmsley
Creator of...

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