Why WebM files don't play everywhere
WebM holds VP8, VP9 or AV1 video with Opus or Vorbis sound — open formats used by browsers, screen recorders and many download sites. iPhones' Photos app, older Windows apps, many TVs and video editors expect H.264 in MP4 instead, so they refuse WebM or play it without sound.
Converting re-encodes the picture to H.264 and, where the browser can, the sound to AAC: the combination every device plays.
Screen recordings without a length
WebM files recorded in a browser often don't store their duration, so players show 0:00 or can't jump ahead. The tool scans the file to find the real length and writes it into the MP4, so the result can be seeked normally.
Quality and size after conversion
VP9 and AV1 are more efficient than H.264, so the MP4 is encoded at about 1.3 times the source bitrate (within the High quality level) to keep the same look; the file can grow a little. Screen recordings with little motion stay small.
If size matters more than compatibility, compress the MP4 afterwards or use Target size in the Video Compressor.
When the sound stays Opus
Firefox and some Linux browsers can't create AAC sound. There the sound is kept as Opus inside the MP4, which current browsers, VLC and recent phones play; the result notes say so. Use Chrome, Edge or Safari for AAC.