From http://jigsaw.w3.org/css-validator/BUGS.html

''
While I was using the very useful CSS validator, I noticed something strange, 
perhaps a little bug... 

With the following style definition : 

BODY {
   background-position: center 90%;
 }

I would say that this definition is wrong because I should not mix "center" and 
a percentage. 

But the fact is that no error or warning is reported, and this is the result of 
the validator : 

BODY {
      background-position : null null;
}
''