MP4 is a box, H.264 is what's inside
Nearly every confusing thing about video formats comes from conflating these two layers.
4 min read
In shortThe same .mp4 can hold very different codecs"My player won't open it" is usually a codec problemRepacking is lossless; converting is not
A container holds streams. A codec compresses them. Separate the two and format questions stop being mysterious.
Why WebM is smaller
WebM usually carries VP9 or AV1, which are 20–30% more efficient than H.264 at the same visual quality. The saving is real, and so is the compatibility risk in older editors and TVs.
Muxing versus converting
Moving streams into a different container is fast and lossless. Converting between codecs re-encodes every frame, takes real time, and throws away detail permanently.
What to pick
MP4 unless something specific in your workflow wants otherwise. Choosing the right container at download time avoids a conversion you can't undo.
Put it to use — paste a link and see the quality list.Open the downloader