A web color taken from X11
In CSS, the keyword greenyellow gives #ADFF2F. The name comes from the X11 color list, which CSS adopted along with most of its other color words. The CSS Color spec lists it with the rest. It is a light, very bright green, a paler tint of CSS chartreuse, #7FFF00.
Easy to mix up
CSS also has yellowgreen, a different and darker color, #9ACD32. In code, only the word order tells them apart.
Crayola has made a Green-Yellow crayon since 1958, but it is a pale yellow close to #F1E788. It looks nothing like the web color.
Black text on #ADFF2F has a contrast ratio of 17.12 to 1. White text on it is very hard to read, at 1.23 to 1.

