Short answer: WebM for Chrome, Firefox, Edge and OBS; ProRes 4444 for editing software; HEVC with alpha for Safari and iOS; PNG sequences for game engines and animation tools.
By destination
| Destination | Use | Avoid | Details |
|---|---|---|---|
| Chrome, Firefox, Edge | WebM (VP9 alpha) | H.264 MP4 | Website guide |
| Safari, iPhone, iPad | HEVC with alpha, or a PNG still | WebM (alpha ignored) | Safari guide |
| OBS overlays and stingers | WebM (VP9 alpha) | Chroma key on a transparent file | OBS guide |
| Premiere Pro, After Effects | ProRes 4444 | WebM | Editors guide |
| DaVinci Resolve, Final Cut Pro | ProRes 4444 | WebM | Editors guide |
| Game engines, sprite tools | PNG sequence | Any lossy video | Video to PNG |
| Anything, as a check | — | Plain H.264 MP4: no alpha | MP4 guide |
What this is based on
We tested the file formats themselves: every export and conversion here is decoded and checked for real transparent pixels, and transparent WebM playback in Chrome. The browser and application rows follow each vendor’s documentation and long-standing behaviour at the time of writing; versions change, so test in the exact app you deliver to.