[css-scroll-snap] snap points on pseudo elements

[frivoal]

In http://www.w3.org/mid/C65090CE-632C-4606-93A9-80F358A15510@rivoal.net, @tabatkins and I argued about snap points and pseudo elements. This was not resolved.

but I'd also be in favor of making it apply to all pseudo elements. As long as something has a box, we should have all we need to make things work.

Yeah, but it has to be tree-abiding, or else it's unpredictable how
many snap-points it generates. This rules out ::first-letter and
::first-line, as well as all the highlight pseudos like
::spelling-error. This is the same reasoning that rules out
background-image and friends - it's unpredictable how many boxes the
pseudo-element will generate, as it depends intimately on the markup
it's applying to, and so we avoid any property that depends on or
visually exposes the number of boxes.

I'm not sure I follow. While these pseudos do weird things to the tree
(making it no longer a tree), how do they create more than one box? Or do
you mean the boxes generated by their children? Why would that matter
for a snappoint?

Also we do apply background image to non tree-abiding pseudos:
http://jsbin.com/nowose/edit?html,css,output

I mean, yes, the non tree abiding pseudos are weird in general, and
there's all sorts of issues with them, but could you clarify a bit
what causes problems with regards to snap-points?

[fantasai]

The reason we don't want to do this is because it is additional implementation and testing work for no clear use case. If at some point we introduce a pseudo-element that clearly needs to support scroll-snapping or there is a demand from the authoring community to add scroll-snapping to an existing pseudo-element, then we can add scroll-snapping to that specific pseudo-element.

We don't even blanket-whitelist backgrounds. We are not going to blanket-whitelist scroll-snapping.

[fantasai]

https://lists.w3.org/Archives/Public/www-style/2016Mar/0353.html

[frivoal]

Didn't remember that we had a resolution for this, sorry.
For the record, I'm OK with the conclusion.