Unknown pseudo-element or pseudo-class ::selection [selection]

Question

Unknown pseudo-element or pseudo-class ::selection [selection] shows the error while validating in w3c validation and i have also tried in css level 3 but not solved.

please reply how to validate it in w3c.

Answer 1

https://developer.mozilla.org/En/CSS/::selection

Right now, the ::selection CSS pseudo-element is not in any specification on the standard track.

The W3C validator probably won't validate something that is non-standard.