Convert MP4 to WebM

Neither. MP4 and WebM are containers, not codecs. This tool re-encodes the video and audio streams to codecs WebM supports (typically VP9 video and Opus audio) and repackages them in a WebM container.

Re-encoding to a different codec involves some quality loss, since the video is decoded and recompressed rather than copied byte-for-byte. It's usually not noticeable at normal viewing sizes.

MP4 (H.264) has the widest compatibility. It plays natively on virtually every device, video editor, and older browser. WebM is well-supported in modern browsers and is common for web embeds, but if you need a file to open reliably in unfamiliar software, MP4 is the safer choice.

Yes, but large videos may be slow or use significant memory on phones. Try a smaller file first if it stalls.