Embed local videos with <video> tag and get local poster within vault

Things I have tried

I’ve searched both this forum as the subreddit and of course through search engines. I’ve tried various other options (see below), but nothing works.

What I’m trying to do

I want to embed a video I have in my vault and make it look fancy with a poster.
So I’d like to use <video> with ‘poster’-attribute. I can get it to work using an URL from an online source, and I can get it to work using a fixed local address using “app://local”

The problem with that last option is that it doesn’t sync across my devices, so I’d like to use a less ‘fixed’ option.

Any ideas to get this to work?