[css-pseudo][css-lists] Allow counter-* in ::marker

[Loirooriol]

The ::marker pseudo-element can use

::marker {
  content: counter(my-counter);
}

However, something like counter-increment: my-counter won't work since counter-increment doesn't apply to ::marker. Gotcha, it has to be set in the list item instead.

So I think counter-increment, counter-set and counter-reset should just apply. Counters don't affect layout so there doesn't seem to be any problem.

[fantasai]

My current thoughts on ::marker's property list is, for simplicity, until we have a layout model for 'outside', we keep it as short as possible while satisfying the main author use cases and implementer concerns. Is there a particular use case you have in mind?

[Loirooriol]

I just wanted something like

ol {
  counter-reset: my-counter;
}
::marker {
  counter-increment: my-counter;
  content: "[" counter(my-counter) "]";
}

Sure, I can set counter-increment: my-counter on the li, but then I need another rule :)

Or I could forget about my-counter and just use counter(list-item). However, list-item is not an actual counter in Chromium, so it has some bugs. For example, until recently, it used to be one unit greater than the correct value... https://crbug.com/796961

So it's not a big deal, but it seems unexpected to me that, given ::marker can read counter values, it can't change them. And changing counters seems completely independent of the outside layout model.

[fantasai]

@Loirooriol Why wouldn't you just set the counter-increment on the li? Wouldn't that make more sense anyway?

[Loirooriol]

@fantasai I don't know, when using counters in ::before, it's not uncommon for me to increment them in ::before too, this avoids needing an extra rule. So it just seems a bit unexpected to me that I can't do the same for ::marker.

[LeaVerou]

Seconded @Loirooriol, I also often increment in ::before as well. That said, I'd consider this pretty low priority.

[Loirooriol]

@fantasai I think your reasoning from #6005 also applies here:

Probably should apply, mostly about style computation:

• transitions
• animations
• counters
• quotes

Since they are mostly about style computation rather than layout, they should be fine in ::marker. Transitions and animations are already allowed, counters and quotes are missing. But yeah, low priority.