A video filename tells you its container suffix, not the complete encoding recipe inside. MP4, MOV, and WebM are containers that can carry different combinations of video and audio codecs, metadata, and track settings. Two files ending in .mp4 may behave differently in the same browser.
Container and codec are different things
The container groups media tracks and metadata. A codec compresses and decodes a track. The browser has to support both the container details and the codecs used by the file. A file picker can accept an extension without being able to play that file.
MP4 and MOV
MP4 is commonly used for web delivery, but codec profile, pixel format, audio codec, and metadata still matter. MOV is a container often encountered in camera and editing workflows; browser playback varies with the tracks inside it. Do not infer a file's frame rate or color properties from the extension alone.
WebM and this site's clip recorder
Framewell's clip capture writes WebM when the browser exposes a supported MediaRecorder format. The recorder captures playback and may re-encode it. It is not a general conversion pipeline and does not create MP4. Check the resulting audio, timing, and quality in a media player before using it elsewhere.
Choose a test that answers your question
Use the Video File Inspector for decoded duration and dimensions. If you need exact codec, frame rate, or stream metadata, use a dedicated media inspector or your editing application. Test a short sample on the destination platform before processing a long project.
Practical rule: use codec and platform specifications, not just the filename extension, when compatibility is important.