[selectors] Proposal: :only-of(selector list)

Hi,

It looks like it would be useful to include :only-of(selector list) in Selectors Level 4.

It is already possible to achieve the same behavior by using :nth-child(1 of selector list):nth-last-child(1 of selector list) but that is significantly uglier. Repeating the selector list is error prone when updating the stylesheet and the specificity is doubled.

The selector :only-of(selector list) would behave like :nth-child(1 of selector list):nth-last-child(1 of selector list) but with lower specificity (accounting for the selector list once instead of duplicating it). The definition of :only-of-type is pretty much the same.

The implementation is trivial based on the existing :nth-child() and :nth-last-child(). The single :only-of() is likely to be more efficient than :nth-child():nth-last-child() in most implementations.

Benjamin