With FFmpeg builds being produced for browsers[1], it'd be great see this working in a web app. I tried making one a few years back, but it was a little early. It only worked in Chrome, on a desktop, and with smallish MP4s at the time[2].

[1] https://github.com/Kagami/ffmpeg.js/ [2] https://zvakanaka.github.io/vidslicer/