XML : Validate xml attributes of unknown element with xsd

I'm using an xsd to validate xml files that may have several unknown xml elements. I know that xsd supports the "any" element for processing unknown xml elements. The problem is that I have to validate the attributes of these unknown elements, and I have not been able to find a way to do so.

Xml dummy example:

  <root>     <aaaaaa gui-name="my name" group="my group" chart="my chart" />     <nnnnnn gui-name="my name" group="my group" chart="my chart" />     <yyyyyy gui-name="my name" group="my group" chart="my chart" />     ...     ...  </root>    

The xml elements are unknown but their attributes are well known. How can I validate the attributes of these unknown elements?

Thanks in advance.