Having a video with a black background inside a container with mix-blend-mode: plus-lighter is an awesome idea! I do not know if this is a common way to have a "transparent" video, but I am definitely going to be using it a lot.
When flash met it's untimely death, the advertising creatives that I worked spent 10 years asking for solutions that involved transparent video in html; we had to tell them no.
I hadn't really considered that browser blend modes have turned that party back on.
I also, on the topic of tech details, make extensive use of the new scroll-timeline stuff which lets you tie CSS animations to the scroll position. it has only landed in chrome at the moment (as of maybe February) so I just use it as a progressive enhancement but it's very cool and performant.