The DjVu.js Viewer demo
DjVu.js is a program library for working with
.djvu files online.
It's absolutely open source and you can see the source code on GitHub
Also there is the DjVu.js Viewer, which allows you to view .djvu files online right in the browser.
It's written in ReactJS and powered with DjVu.js library.
Here you can see the Viewer and try it out. If you don't have a djvu file you may view the examples - just click on one of the following links.