Playing video/audio offline?

Question

Audio

In theory, you can compress 100 mb to 10 mb and download it to the user's device, unpack it before playback, if he turned on this option in the settings. Let's say if the user plans to watch some content and then we preload it. For example, if he watched / listened to 80% of the video / audio, we also download it so that he can use the application offline. Naturally, it will be necessary to give the opportunity to manage downloads and toggle the feature.

An application for the web and mobile devices, so I would like to know the advice of those who have done something at least approximate.

Any ideas on how to implement this correctly?
What difficulties will arise?

Answer 1

In theory, you can compress 100 mb to 10 mb

Wow.

What difficulties will arise?

The main difficulty is to find a codec that can compress video by 10 times.

Any ideas on how to implement this correctly?

Put together a team of professional mathematicians and sound engineers, invest a couple of million dollars and tell them to come up with such a codec.
There are no other solutions.