[barkingcat]

Yes, the "processing" refers to the name of a programming language / programming environment originally founded / created by people at the MIT Media Lab.

It is not referring to "processing" anything as a verb, but the name of the programming language outlined at https://www.processing.org

processing.js and this p5js are interpreters for the processing language built in javascript (so that people can write processing scripts in javascript).

You probably haven't heard of it in browser ui/development because it's not specific to the browser.

Many people use it to build interactive art environments both virtually and in the real world (tied to things like servos and light machines, etc) - and a lot of people use it to teach interactive graphics to kids or newcomers to programming.

So it's a totally different thing than what you are thinking of. It doesn't compete at all with any of the graphics libraries because it's for a different purpose.