Currently only works on 'old reddit', such as using Reddit Enhancement Suite or old.reddit.com.
Contents of file dist\index.js in this repo can be copy and pasted into the console on a Reddit page with an .mp4 video such as https://www.reddit.com/r/aww/comments/12ujv9d/employee_of_the_month/ to download the video.
To use this more easily, save it as a boomarklet. Create a new bookmark, and enter javascript: followed by pasting the entire script as the url target. This allows you to simply open a reddit page and click on the bookmark to download the video.
Build your own script with yarn parcel build src/index.js, then the output will be in dist\index.js.
This project essentially just gets the separate video and audio files that Reddit uses to play a video, and packages them into the same video container with ffmpeg-wasm.
Thanks to https://github.com/Unnoen for helping with getting ffmpeg bundled and running in the browser.