Quote from the book "Письма к брату Тео. Первое полное издание"

Suitable for smartphones, Android tablets, e-readers (except Kindle) and many programs

Accessible from nearly any device

Works on any computer

Works on any computer

Optimized and suitable for smartphones

Suitable for Android apps

Suitable for iOS devices (iPhone, iPad, iMac) and most reading apps

Perfect for iPhone and iPad

Development of FB2 format