For policy reasons, Wikimedia Commons doesn't want to host videos using proprietary file formats, like MP4 (https://commons.wikimedia.org/wiki/Commons:Requests_for_comment/MP4_Video). UploadWizard used to use the Firefogg extension (https://firefogg.org/) for on-the-fly transcoding of MP4 files to OGG in the user's browser. But due to changes to how Firefox handles extensions, this is soon to stop working (https://blog.mozilla.org/addons/2016/11/23/add-ons-in-2017/) and it seems already impossible to install the extension.
With modern browsers' fast JavaScript engines and HTML5 file APIs, it should totally be possible to implement transcoding in JavaScript, without shelling out to external executables, like Firefogg did. As a bonus, that would work on more browsers than Firefox.