Error with CSS validator

Hi,
 
I'm trying to check my CSS in this page:
http://w3.oqlf.gouv.qc.ca/formulaires/toponymie/index3.html .
I went through the HMTL validator and everything is fine. But each time I
tried CSS validator, I got this result : 


Target: http://w3.oqlf.gouv.qc.ca/formulaires/toponymie/index3.html


Please, validate your XML document first!

Line 1

Column 1

Content is not allowed in prolog. 

What does "Content is not allowed in prolog" mean?

Stephanie