WebM and MP4 are container formats, and there is barely any advantage to using one over the other. It is more important what codec is in them.
And for simplicity, compatibility, and encoding speed, I decided to just do h264/mp4.
VP9, the codec usually in WebM, is about 20%-30% smaller but takes about 10x as long to properly encode. And you likely need a h264 fallback for old iPads or similar devices.
This extension solves the “I can now upload videos from my phone or Adobe Premiere” issue and creates pretty good results for that. Good enough for 90% of cases and definitely better than nothing, without any additional infrastructure requirements