Sometime in about mid-June, 2015, the w3c CSS validator stopped accepting CSS :after selectors that use the "content:" keyword with the "attr(<attribute-name>)" value:
  :after { content: attr(<attribute-name>); }

Though files with that selector used to validate properly, and still work in all browsers, the w3 CSS validator has started giving the error message:
  Value Error : content Parse Error attr(<attribute-name>) 

Example:
This CSS statement
  .pageno:after { content: attr(title); }

gives this error:
  Value Error : content Parse Error attr(title)