Free WebM to MP4 converter, no upload

WebM holds VP8 or VP9 video and Opus or Vorbis audio. An MP4 could technically carry them, but iPhones, PowerPoint and most editors would not play the result. So the video is re-encoded to H.264 and the audio to AAC. For a 720p recording that takes about as long as the clip, because the encoder runs on one thread in the tab.

Yes. Browser recorders write WebM as a live stream and never go back to store the duration or a seek index. The MP4 is written with both, and with the index at the front of the file so it starts playing before it fully loads.

H.264 MP4 has no transparency, and this page does not carry an alpha channel across, so transparent areas become solid. Keep the original WebM wherever you need the see-through background.

Not usually. Browser recorders encode fast and leave size as an afterthought, so the H.264 file often comes out smaller; the worked example on this page shows one real recording. If you need it smaller still, pick Smaller, or 720p under More options.

500 MB and 10 minutes. Everything runs inside one browser tab, which has a fixed memory budget, and the encoder runs on a single thread. A rewrap takes seconds; a full re-encode runs at about real time for 720p and several times the clip length for 1080p. For anything bigger, a desktop encoder such as HandBrake is the better tool.