Unknown pseudo-element or pseudo-class: selection [choice] shows an error when checking in w3c validation, and I also tried in css level 3, but did not solve it.
answer how to check it in w3c.
https://developer.mozilla.org/En/CSS/::selection
Right now, the :: select CSS pseudo-element is missing from any specification on the standard track.
The W3C validator will probably not check for something non-standard.