MP4 is probably the best format to use, it’s supported by all modern browsers.
There’s no code required from the poster to embed the video, as long as you have the video filetype set in the authorized extensions setting you can upload it or drag it into the composer as if you’re posting an image.
You can also post a link to an externally hosted MP4 on its own line, and it’ll be embedded
For example, putting this link on its own line
https://i.imgur.com/cLEzWIh.mp4
Embeds the video like this:
The video is embedded using the HTML5 <video> tag, which uses your browser’s native player.