This is a demo page for the responsive-video HTML web component, which wraps a standard responsive video element and makes it respond to media changes that happen after pageload (which it would not do by default), while syncing the video's timecode as well.
The web component is only bound after a feature test that makes sure the browser does not support reassessing video source media changes after page load. This is just extra cautious, as at time of writing, no browsers do that and there's no known plan for that to change.