I made this demo 10 years ago when the browser landscape was pretty different.
If I was going to do this today, I'd just use a video format that supports alpha, which currently means HEVC for Safari, and VP9 elsewhere. Here's a good article on the subject
You might still go for a solution like this if you want to support old IE, but in that case you'd be better off using webgl to do the image processing.
Random background colour - Stop - Start - Toggle Processing