How to make a video recording on a web page and save it to your device?

Question

Python

Greetings. There is a raspberry pi 3 with a raised python web server and 2 images are streamed via mjpg, one from a regular camera, the other from a thermal imager. It is possible through html and css to overlay an image on top of each other or show it side by side. The whole thing is hung on a quadrocopter, and it takes off and gives a picture via wi fi.
And now viewing the picture works, but how to make a video recording, so that it would be immediately added to the mobile phone, well, or just stored in memory and then saved? I look in the direction of the client recording on the frontend ... The quality should be no higher than 640x480 and the frame rate should not be higher than 7 frames per second.
But if it doesn’t work out, I’ll make an entry on OpenCV and throw a link into the web interface.
The recording needs to be done on the android, I will consider options for building the application on cordova / phonegap or similar ...
In general, kick where to dig :) Thank you!
PS option with third-party screen recording software, please do not offer ...