MP4: The Universal Standard
MP4 with H.264 video and AAC audio is supported by every major browser, operating system, and device on the planet. It's the safest choice for streaming. H.264 compression is mature and efficient — a 90-minute film can be encoded at 1080p in under 3GB and still look excellent. The downside: H.264 is a patented codec, meaning some open-source projects avoid it. For streaming purposes, this doesn't matter at all.
WEBM: Better Compression, Narrower Support
WEBM uses VP9 (or the newer AV1) video codec, which achieves noticeably better quality per megabyte than H.264 — typically 30–50% smaller files at equivalent visual quality. The catch: WEBM is not supported in Safari at all for VP9 streams. AV1 support is gaining ground but still not universal. If your audience is primarily on Chrome or Firefox, WEBM is a genuinely superior choice. If Safari support matters, stick with MP4.
Which Should You Use with FluxPlays?
Both formats work in FluxPlays. If you're pasting a link from an existing source, the format is determined for you. If you're encoding your own content, use MP4 for the widest compatibility. Use WEBM/VP9 if you know your audience uses Chrome and you want to save bandwidth. FluxPlays detects the format automatically and uses the appropriate decoder path.