Why Dark Gray Is Brighter Than Gray in CSS

On the other hand, “dark-gray” was left alone as it did not exist in HTML or CSS at the time so it was brought in from the X11 definition where it is defined as 66% shade of gray. As a result, the combined CSS color list that prevails on the web today renders “dark-gray” as a significantly lighter shade than “gray” because “dark-gray” came from X11 while “gray” came from HTML. Even today in the latest CSS draft the color “dark-gray” produces a lighter shade than the color “gray”, and just to prove naming things is hard both grey and gray are valid color names.

Source: medium.com