[selectors4] Time-dimensional pseudos need another spec to define a timeline

Hi,

I find the current definitions of :current and other time-dimensional 
pseudo-classes a bit misleading:

> The :current pseudo-class represents the element, or an ancestor of
> the element, that is currently being displayed.


There are tons of slideshow systems out there based on HTML and 
JavaScript, where each slide is an HTML element. At any time, there is 
one slide being displayed, which matches perfectly the definition of 
:current.

But :current does not match any element in this case, because web 
browsers do not implement it that way. (They don’t really know about the 
JavaScript-constructed "timeline" of slides.)


I suggest clarifying that these pseudo-classes depend on a timeline, and 
do not match anything unless the UA supports another spec (such as 
WebVTT) that defines such a timeline.

(This is similar to how Selectors depends on other specs to define "the 
language of a element" for :lang().)

Cheers,
-- 
Simon Sapin