The extension is only part of the story
MOV and MP4 are containers: they hold video, audio, timing, and other information. The video itself is encoded using a codec such as H.264 or HEVC. Two MOV files can therefore require very different decoder capabilities.
Your browser, operating system, hardware, and the tool’s media parser all affect whether a file can be read. Renaming a file from .mov to .mp4 does not change its codec or repair a damaged file.
Try a small, deliberate diagnosis
- Check that the file plays in your usual video player and is fully downloaded.
- Try a current browser on a desktop device. A browser update can change available decoding support.
- If the file is HEVC, 10-bit, HDR, or a professional editing format, expect support to vary.
- Export a short H.264 MP4 copy from a trusted local editor or converter, then try that copy.
- Keep the original file. A compatibility copy may change color, compression, frame timing, or metadata.
Pay attention to color and orientation
Some phone recordings carry rotation metadata rather than physically storing every pixel in its display orientation. Check that the preview and exported image have the correct orientation before processing the full selection.
HDR and wide-gamut sources can look different after browser decoding and canvas export. If accurate color is part of your deliverable, compare the output in a managed desktop workflow. Choosing PNG does not by itself guarantee HDR preservation.
What ClipStills currently promises
H.264 MP4 and VP8/VP9 WebM are the priority input families. Other supported combinations depend on the browser’s decoders. The compatibility page distinguishes intended support from completed device testing.
There is no server conversion fallback in this release. If the browser cannot decode a source, use a local desktop workflow or make a compatible copy instead of repeatedly submitting the same job.