A lovely website (or web book?) dedicated entirely to colour contrast, complete with interactive illustrative widgets.
A comprehensive guide for exploring and learning about the theory, science, and perception of color and contrast.
This is a truly wonderful web page! It’s an explanation from first principles of how cameras and lenses work.
At its most basic, it uses words which you can read in any browser. It also uses images so if your browser supports images, you get that enhancement. And it uses interactive JavaScript widgets so that you get that layer of richness if your browser supports the technology.
Then you realise that every post ever published on this personal site is equally in-depth and uses the same content-first progressive enhancement approach.