(1) Pseudo-element notation requires **double** colon notation. For example: FOO::selection{}. The only exception is that single colon is permitted for the 4 pseudo-elements from CSS 2.1, namely :first-line, :first-letter, :before and :after.

Therefore, because ::selection is a new (and seemling the *only* new ) CSS 3 pseudo-element, the double colon notation is therefore required for the ::selection{} selector. And it is therefore incorrect when the Validator accepts as valid the following: 

       FOO:selection{background:lime} 

(2) LIKEWISE, pseudo-class notation always  requires a **single** colon, for example:  FOO:link{}.  

Therefore, it is incorrect when the CSS Validator stamps the following selectors as valid:

    ::link,::visited,::active,::focus,::hover,::target, ::root,::first-child,::last-child,::checked, ::enabled,::disabled,::indeterminate,:not(::link)  {}

(3) For reference: The only times the CSS validiator correctly rejects double colon notation for pseudo-class selectors, is for the following (note that all these selectors include a paranthesis):

    ::not(*),::nth-child(1),::nth-last-child(2),::nth-of-type(2),::nth-last-of-type(2){color:red;}

(4) Note that :not(::link) currently gets accepted, whereas ::not(::link) does not.

See CSS 3 about pseudo-elements: http://www.w3.org/TR/css3-selectors/#pseudo-elements