[css-pseudo] add "nth-*" pseudo element

[carnoxen]

Pseudo class has "nth-*" selectors , but pseudo element don't. How about adding "nth-*" selectors?
Here's what @chriscoyier says: https://css-tricks.com/a-call-for-nth-everything/
At least, I want the pseudo element spec to implement the selectors:

• ::nth-line() / ::last-line / ::nth-last-line()
• ::nth-letter() / ::last-letter() / ::nth-last-letter()

Once implemented, it would be powerful to design the thing like code block, poem, etc without code complexity.

[frivoal]

This sits at the uncomfortable intersection of useful and freaking hard to implement.

The existing ::first-line and ::first-letter already cause lots of headaches, and implementers are not looking forward to more of that complexity.

That said, the kind of things you want to do with ::nth-line might end up being addressable with:

thing {
  max-lines: 1;
  continue: fragments;
}
thing::nth-fragment() {
}

This is and early / unstable draft, but have a look here for more details:
https://drafts.csswg.org/css-overflow-4/#fragmentation
https://drafts.csswg.org/css-overflow-4/#fragment-pseudo-element

As for ::nth-letter, I am not aware of any substitute, but neither am I aware of strong use cases. If you want this explicitly (instead of just for completeness sake), could you give some examples of where you'd use it and why existing CSS isn't good enough? There's one in the blog you linked to, but do you have more?

[fantasai]

Closing out per #745 (comment)
It's possible we might revisit this idea in the far far future, but for now, based on past WG and implementer discussions, I think it's unlikely to happen.