[css-contain-2]: Pause media playback in iframes hidden by content-visibility:hidden

https://drafts.csswg.org/css-contain/#content-visibility

We have a use case where we would like the ability to pause playback of video when we mark a cross-origin <iframe> as being content-visibility: hidden.

Eg, we include an iframe to youtube video, which the user then plays. After playing, the user becomes disinterested and scrolls on to the rest of the document. After the users is far enough away, we manually mark youtube iframe with content-visibility: hidden. Ideally, this would pause the playback so it's no longer taking resources, audio would stop playing, and the state would be paused. If the user were to scroll back to the youtube iframe, they'd find it in the paused state at the exact point they scrolled away.

In order to achieve this currently, we actually remove the entire iframe from the document. This guarantees the media will stop playback, but it also loses the current track position. If the user scrolls back to the the youtube iframe, they'll find it reset to the 0s mark, which isn't a great experience for them.

/cc @chrishtr

Start with the linked CSS Containment specification section on content-visibility and review the issue's iframe and media-playback use case. Determine how hidden cross-origin iframes should affect playback while preserving their position; done requires an agreed specification change, not an implementation or named test.