Two containers, two different jobs
MP4 is the universal default — every phone, camera, and editing app exports to it, and it plays everywhere without a second thought. WebM was purpose-built for the open web: it's the format YouTube and most modern browsers use internally for streaming, because its codecs (VP8/VP9, and now AV1) compress more efficiently at a given quality than the older codecs MP4 usually carries.
That efficiency gap matters most when you're serving video directly from your own site rather than through a third-party player — a WebM file at the same visual quality can be noticeably smaller, which means faster page loads and less bandwidth cost.
When this conversion actually pays off
If you're embedding video with an HTML5 `<video>` tag on your own site, WebM alongside an MP4 fallback is the standard modern pattern — browsers pick whichever they support best, and WebM usually wins on file size. It's also the right choice for background videos, product demos, and any autoplay muted loop where every extra megabyte directly slows down your page.
If you're just sharing a file with someone or uploading to a platform that re-encodes anyway (most social platforms do), the conversion buys you little — keep the MP4 and save yourself the extra step.
What actually happens during conversion
Converting isn't just renaming a file extension — the video stream gets fully re-encoded from MP4's typical H.264/H.265 codec into VP8 or VP9, and audio gets re-encoded into Opus or Vorbis. Because this is a genuine re-encode, there's a small, usually invisible quality trade-off each time, which is normal for any lossy-to-lossy conversion and not a sign anything went wrong.
This also means conversion takes real processing time proportional to the video's length and resolution — a quick clip finishes in seconds, but a long 4K file will take noticeably longer since every frame is being decoded and re-encoded from scratch.
Browser support is no longer the concern it used to be
A few years ago, WebM support was patchy enough that you needed an MP4 fallback for Safari and older browsers. That's mostly resolved now — every major browser plays WebM natively — but the fallback pattern is still good practice for older devices and unusual embedded contexts you don't control.
The safest approach for a public-facing site is still to offer both: WebM for the size win on modern browsers, MP4 as the fallback nobody will ever actually need but costs nothing to include.