[css-view-transitions-2] Define the timing for capturing old Document state with respect to events on new Document

[khushalsagar]

#8785 and #8805 clarify the syntax of the events which are fired on the old/new Document during a transition. The timing of these events with respect to a navigation's lifecycle needs to be clarified.

Here are some constraints:

• The event dispatched on the old Document must be followed by a render to capture its visual state.
• The event dispatched on the new Document must be after the above event, since it passes contextual information.
• The event dispatched on the new Document show allow addition of render-blocking resources.

[noamr]

See #8805 (comment).

• Not sure we should send a new type of event on the old document at all. Let users customize their exit state on navigate/click events rather than rely on a precise processing model.
• The JS in the new document should be activated after all the JS in the old document is complete.
• See [css-view-transitions-2] Script event on new Document for cross-document ViewTransitions #8805 (comment). A place to do this would be a regular script at the end of the head.

[noamr]

I think this can be closed @khushalsagar ?