[css-view-transitions-1] Clarify the list of pseudo-elements that can be followed with :only-child

[nt1m]

I'm guessing all the functional pseudo-elements can be followed by :only-child, but there are some clarifications I'd like to see:

1. Does it apply to ::view-transition ? (It would always apply if yes?)

2. Does it apply to ::view-transition-group(...) / ::view-transition-image-pair(...) ? (I'm guessing yes)

3. Is an element with only with display: none siblings counted? (I'm guessing no)

I guess I can go dig into Chrome's source to get the answer but it would be nice to have this explicitly put in the spec.

[nt1m]

@khushalsagar @noamr @vmpstr @bokand

[nt1m]

I admit I also have trouble understanding how :only-child works for ::view-transition-old/new(). Would it be possible to define those in terms of a selector matching function and the namedElements map?

[nt1m]

It also occurs to me that there's no definition for group/image-pair (even though that one is more obvious)

[bramus]

I admit I also have trouble understanding how :only-child works for ::view-transition-old/new(). Would it be possible to define those in terms of a selector matching function and the namedElements map?

I think the spec is already pretty clear on this. For ::view-transition-old() it reads “it is omitted if there’s no “old” state to represent.”. For ::view-transition-new() it has a similar thing in place.

In the intro to the VT pseudos it shows a diagram of the tree-structure of the pseudos, which makes it clear that ::view-transition-old() and ::view-transition-new() are direct children of ::view-transition-image-pair(). That part also reads “Either ::view-transition-old() or ::view-transition-new() are absent in cases where the capture does not have an “old” or “new” state.”

With only one of the ::view-transition-old()/::view-transition-new() pseudos possibly being prevent, authors can use :only-child to detect whether there was only a new or only an old snapshot for that specific v-t object. Based on that, they can do different animations.

See https://codepen.io/bramus/pen/mdabWzr for a demo: when you add a card there is only a ::view-transition-new() pseudo which does a slide-down+fade-in animation. When removing a card there is only a ::view-transition-old() pseudo which does a slide-up+fade-out animation.

[nt1m]

Either ::view-transition-old() or ::view-transition-new() are absent in cases where the capture does not have an “old” or “new” state.

Oh I totally missed that, thanks! Would be good if the spec was more explicit about which property on the captured element struct this relates to. Right now I assume it's "old image" and "new element", but it could be any of the "old" properties :)

[noamr]

Either ::view-transition-old() or ::view-transition-new() are absent in cases where the capture does not have an “old” or “new” state.

Oh I totally missed that, thanks! Would be good if the spec was more explicit about which property on the captured element struct this relates to. Right now I assume it's "old image" and "new element", but it could be any of the "old" properties :)

We can add a clarification, why not. Want to have a go at a PR @nt1m ?