webm vs mp4: which one to use.

WebM and MP4 are containers: the box that holds the video track, the audio track and the timing information that keeps them in sync. The codec is how the pictures inside are compressed. Most confusion about the two formats comes from mixing these up.

Neither, on its own. At the same bitrate, the same clip comes out the same size whichever container it is in; the container overhead is a few kilobytes. What changes is how good the picture looks at that size, and that is down to the codec.

Newer codecs need fewer bits for the same quality. VP9 and HEVC generally look noticeably better than H.264 at the same bitrate, and AV1 better again, at the cost of much slower encoding. VP8 is an older design, broadly in the same class as H.264.

• MP4 with H.264 plays on practically every phone, browser, TV and editing app made in the last fifteen years. It is the safe default.
• WebM plays in Chrome, Firefox and Edge. Safari added WebM support in later releases, so older iPhones and Macs may refuse to play it.
• Some sites only accept one. Image boards and some forums ask for WebM specifically; many social apps expect MP4 and convert anything else.

Going from an H.264 MP4 to WebM means re-encoding, because WebM does not carry H.264. Every re-encode loses a little detail, so start from the best copy you have, not one that has already been compressed for sharing.

The Basketweaver browser converter encodes VP8 WebM, which the in-browser encoder supports reliably on every desktop browser, and caps the file at 4 MB.