WebM to MP4: Compatibility Fixes and Size Trade-offs
The WebM plays perfectly in your browser, then turns into a black screen in your video editor and a dead file in chat. The container is fine - the codecs inside are the problem.
Why WebM fails outside the browser
WebM is a container that usually carries VP8 or VP9 video (newer files may use AV1). Chrome and Firefox ship decoders for all of them, so playback is seamless on the web. Most editors, chat apps, older Android builds, TVs, and streaming boxes only hard-decode H.264/H.265 - they hit a VP stream and give up.
The diagnostic is not whether the file plays, but where it plays. Web in browser, broken locally almost always means a codec compatibility gap, not corruption. Transcoding is the fix; switching players only works around it.
Why the MP4 comes out bigger
VP9 compresses noticeably better than H.264 - the same visual quality typically costs 20-40% less space. Re-encoding VP9 into H.264 therefore grows the file. That is the compatibility tax, not a mistake.
If the goal is editing and sending, accept the size. If you only need to watch it locally, installing a VP9-capable player costs nothing. The reverse direction (MP4 to WebM) is the one that actually saves space.
Three checks after converting
Audio: WebM commonly carries Opus or Vorbis, which gets converted to AAC. Voices sound the same, but spot-check that no section went silent. Duration: compare start and end against the source to rule out demuxing glitches. Subtitles: embedded subtitle tracks are usually dropped - export them separately first if you need them.
After converting, verify resolution and frame rate match the source before deleting anything. Re-encoding is one-way traffic; keep the original WebM as your archive copy.