Kantu allows you to select specific areas of the website to test, and - especially important - the image comparison is error-tolerant. That means that (smaller) changes in image and font rendering do not break the test. So a test case created on the Mac works on Linux and Windows as well. In this respect it is similar to Sikuli (http://www.sikuli.org/ ) - but all inside a Chrome and Firefox browser extension, with no external installations required.

Edit: More info about the choice of WASM for image search here: https://news.ycombinator.com/item?id=17574862