This is the abstract for your specification.

## Synchronizing content in nested browsing contexts

Elements that establish nested browsing contexts such as the `object`, `embed`, and `iframe` elements, contain their own `contentDocument` with its own timeline. In order to synchronize animations in the referenced content with the creator browsing context... use `mediaGroup`? Call it `syncGroup` here?

## The `‘body’` element

Contains the `timelineStart` attribute.