validation of a:after

Hi everybody,

Trying to validate the simple

a:after {
     content: " (" attr(href) ")";
}
(that I can found, for example, in 
http://www.w3schools.com/cssref/pr_gen_content.asp)

on https://jigsaw.w3.org/css-validator, I obtain two parsing errors
(2 	a:after 	Parse Error attr(href) ")";
3 	a:after 	Parse Error } )

I'm not fully able to understand the problem (and didn't find the topic 
discussed in any previous archives).

Hoping not a silly question, thank you in advance for helping.
Andrea S.