Poster images for videos outside viewport should be lazy-loaded

[westonruter]

When a VIDEO with a poster is outside the viewport, the poster image will be downloaded with a medium priority even though the element is not visible:

This being the case, the poster should ideally be lazy-loaded when the video nears the viewport using IntersectionObserver.

Related:

• Improve autoplay for videos outside initial viewport #1590
• Optimization Detective: Implement lazy-loading of background images #1035

[westonruter]

As noted in #10 (comment), the video element's initial dimensions are set to match the poster image's intrinsic dimensions when no width and height are specified. However, if we are setting the width and height based on gathered client-side metrics, lazily-supplying the poster shouldn't cause any layout shift.

[swissspidy]

This would be a big win. Even with preload=none, the browser always downloads the poster image right now, even if it's way outside the viewport.