What is this `ButtonText` keyword?

[sindresorhus]

What is this `ButtonText` keyword?

[arcticicestudio]

The ButtonText is a system color variable defined in the CSS2 (:exclamation:) specification where these colors have also been deprecated.

ButtonText
Text on push buttons. Should be used with the ButtonFace or ThreeDFace background color.

MDN also says:

Not all system colors are supported on all systems. 👎 for use on public web pages.

This is also in the master branch of the origin normalize.css project. It seems like the block you've linked has been copied directly from the Firefox source code.

One of the normalize.css maintainers also said in necolas/normalize.css#481 (comment):

[...] For future reference (who knows if or who will revisit this issue), my concern with outline: 1px solid is what omitting the color on outline does. For instance, when the color is omitted on a border, its default value is the value of the element's color property, which would be the same as using inherit.