@Eric_aus_Chemnitz The videos that @PedalSprell provided were HEVC encoded. HEVC (H.265) is an open standard BUT it is heavily patented. That means there is a licensing fee to decode. Non-profit OSes that are strict in patent handling (most Linux) will not be able to handle this because the cost of paying the license is too much to bear. This is also true of many less common browsers (again the fee is more than they might make per user). Also as a “relatively” new format older hardware will also likely not have support for hardware decoding and software decoding might be too slow or again too costly.
I re-encoded @PedalSprell’s videos and reposted for you as webm. This should work most places though perhaps not on some Apple devices.
In terms of raw capatiblity an MP4 container with H.264 and AAC audio will generally work best. That is also what I would call a proprietary format in that again while the specs are open there are licensing fees involved. However most browsers have worked out complex workarounds and will play H.264 nonetheless which is why the majority of websites tend to use this. Or maybe it is the other way around. It became dominant and hence browsers had to make it work.
H.264 is generally not as small as H.265/HVEC or AV1 but perhaps still safest overall for compatiblity here.
Also in case you are wondering, most sites will not show up these kinds of issues as they tend to reencode uploaded videos anyway picking a format that works everywhere (like H.264) or presenting different versions of the videos to different browsers based on support (no animated “GIF” on X/Twitter is a GIF when your browser received it back from the site, even if it was uploaded as an actual GIF originally). Discourse (which powers are forum) seemingly does not re-encode and gives back the video that was uploaded in the same format it received. Hence these kinds of issues can happen here if you upload videos in less supported formats. Either rehost videos or stick to something you know is widely supported.