Format is not the whole story
MP4 and WebM are containers that hold compressed audio and video tracks. Playback depends on the codecs inside them, not simply the filename extension. MP4 often carries H.264 video and AAC audio; WebM commonly carries VP8 or VP9 video and Opus audio.
Renaming a .webm file to .mp4 does not convert the bitstreams and may make the result less compatible.
Choose for the destination
MP4 with widely supported codecs is often a practical choice for ordinary sharing and editing workflows. WebM is useful for browsers and web delivery when supported. Always check what a specific upload service accepts.
Some browsers can play a format but cannot record or encode that format. Playback support and export support are separate capabilities.
Compression trades size for detail
A lower bitrate generally reduces data but can soften detail or introduce artifacts around motion, fine patterns and captions. Lowering dimensions reduces pixel count and can also decrease the required bitrate. Very noisy or fast-moving clips need more bits to look clean than static scenes.
An already well-compressed source can become larger after re-encoding, especially if the selected bitrate exceeds the original's effective bitrate.
Audio is part of the file
A video export must handle audio tracks, sample rates and browser permissions correctly. Browser MediaRecorder features differ across devices. Some MOV sources cannot be decoded or have their audio captured in a given browser.
Do not publish a finished clip until you verify that both its picture and sound play from start to finish.
Reliable workflow
Pick a supported output format, set a maximum width and balanced bitrate, compress, then play the exported file in the target app. Compare file size and clarity with the source. Test any subtitles or on-screen text, especially at smaller resolutions.
For irreplaceable footage, keep a full-quality master and treat browser exports as delivery copies.
Compare exports fairly
Start with a short clip containing motion, detailed textures and spoken audio. Export at the same width using two supported encoding configurations. Check actual file sizes, playback on the receiving device and whether audio remains synchronized. Do not treat a lower byte count as a win if subtitles become unreadable.
Like still images, video compression has a resolution-versus-quality tradeoff. Our image sizing guide explains the underlying pixel-dimension idea, though temporal video codecs add motion prediction and additional complexity.
Why one browser may behave differently
Browsers expose different sets of MediaRecorder encoders. A platform that can play H.264 MP4 does not necessarily allow MP4 recording. Source MOV files can present separate audio capture difficulties. The format selector should therefore reflect actual encoder support, not an imaginary universal conversion capability.
When the clip is important, preserve the camera master and check the exported file from start to finish before sharing.