When adding/removing regular elements from the DOM, we can use defined terms like pre-insert, and remove.

I don't think similar things exist for pseudo-element trees?

Right now we're using language like "append somePesudoElement to someOtherPseudoElement", but the term "append" isn't linked, so it's a bit of a hand-wave.

@fantasai @tabatkins are you aware of any other specs that have ran into this?

If it isn't a problem generally, we could define a "view transition pseudo-element" dfn that has a "pseudo-element children" list, and say that a view transition pseudo-element must render its pseudo-element children in order.