There are a lot of ways to work with color on the web. I think it’s helpful to understand the mechanics behind what you’re using, and color is no exception. Let's delve into some of the technical details of color on the web.
Weird right? 4-digit hex codes too. They are a way put alpha transparency information into the hex format for colors. You might be seeing it more all the sudden if you use Chrome, because it dropped in 52, and the DevTools seem to be defaulting to showing alpha transparent colors in this format even if you set it originally using something else.