Well, to create the website, it’s going to be HTML, CSS, and JS - everything on the frontend of a website can only be those three things.
For the actual image processing, options would be a library or a backend service - or a combination. Maybe there is already a library you can implement in JS. Or maybe you write one. Maybe there is a backend service. Or maybe you write one. Any library would need to be in JS. A b/e service I would assume would want to be something faster, like C or C++. But since the site you are referencing is doing it in real time, I assumed that it was a library. But then again, there seems to be a lot of network traffic as I’m doing things so maybe there is a back end. Of course, that back end may itself use a library to do what it needs to do.