I made an Astro component for this [1] which does the iframe srcdoc thing [2] (example page with dozens of videos [3]). Most of the code is just TypeScript types and building a big srcdoc string, so easy to repurpose if anybody wants to.

Handling the fallback image has been sitting as an issue in the repo for a while, in favour of just checking it with the Astro dev server when I'm adding new videos, so… yoink‽

[1] https://github.com/insin/astro-lazy-youtube-embed#readme

[2] https://css-tricks.com/lazy-load-embedded-youtube-videos/

[3] https://jbscript.dev/notes/undefined/the-banterbox