Capturing data stream of a JS / Leaflet animation as MP4

Question

How can I capture the datastream of a JS / Leaflet animation and download it to MP4?

I am looking for output that looks something like the smooth path traced in these demos: https://github.com/IvanSanchez/Leaflet.Polyline.SnakeAnim

Their author appears to have done them in ffcast or some screencasting softare.

However, I am looking for an automated solution that can be run as script, ideally one that works on the data stream itself (not the screen), perhaps with a headless browser.

I have tried puppeteer-gif and puppeteer-gif-cast but the best frame rate is jumpy.

I have tried WebRTC-Experiment but it requires me to set manual permissions. Ditto the Screen Capture API mentioned here, though this at least seems to work on the data stream itself.