If startViewTransition is called while a Document is hidden. Or the document is hidden at any point during the transition's lifecycle, we don't do anything special. We should probably explicitly skip the transition if the Document is hidden.

Not sure what to do with iframes. The UA can stop rendering opportunities if the iframe is far enough for the viewport. Should we assume its hidden?

The spec way of when a Document is hidden can be based on this : The step labeled Rendering opportunities prevents the user agent from updating the rendering when it is unable to present new content to the user. If the Document is in this state, skip all ViewTransitions?