Cool—and open source. 9to5 Google writes,

By using WebAssembly, Squoosh is able to use image codecs that are not typically available in the browser.

Supporting a variety of web formats like MozJPEG and WebP and traditional ones like PNG, Squoosh allows you to quickly make your images web-ready. The app is able to do 1:1 visual comparisons of the original image and its compressed counterpart, to help you understand the pros and cons of each format.

[YouTube]