GIF video embedded in a forum post renders nicely but if you want a reasonable resolution, frame rate and colour depth, the file sizes will be huge. MP4 is more compact, but doesn't render at all in the forum (for me anyway) so I can only use it as an attachment.

Before I test every possible video format, maybe a website admin could tell me what works and what doesn't.

Mp4 format works best

MP4 is just a container and don´t tell anything about the codecs used.

That's right But i prefer mp4 videos

I believe you have no clue what you´re talking about.

Nowadays all video content is stored in an MP4 container, which didn‘t says much about used codecs of course. You can search Gooogle sites on their recommandation of codecs+stuff like here: https://support.google.com/youtube/answer/1722171?hl=en-GB