I was surprised that Twitter only lets you upload H.264 still, in 2026.
Even then, it was a crapshoot whether or not the video would look like a pixel collage, and I'd need to delete the Tweet, tweak the video a bit, and try again.
E.g. I remember having lots of issues sharing smaller video files encoded with H.265 via both WhatsApp and Messenger. iMessage doesn't support H.265 (or at least it didn't work for me at the time).
How are you generating video?
How are you playing it back?
AV1 is the future/present: Fully open standard, better than h265. Only downside is hardware support: You need hardware made in the last few years.