Validation of the rule *{background-position:center;} results in output of 

*  {
background-position : center 0%;
} 

as "Valid CSS information". Sure, this is valid, but "center 0%" is not equal to "center". "center 50%" or "center center" would be a correct expansion, while "center 0%" means the same as "center top" which is obviously something different.

Relevant part of the spec: <http://www.w3.org/TR/CSS21/colors.html#propdef-background-position>
>If only one value is specified, the second value is assumed to be 'center'.