Got curious about this.

https://en.wikipedia.org/wiki/X11_color_names#Clashes_between_web_and_X11_colors_in_the_CSS_color_scheme :

One notable difference between X11 and W3C is the case of “Gray” and its variants. In HTML, “Gray” is specifically reserved for the 128 triplet (50% gray)  . However, in X11, “gray” was assigned to the 190 triplet (74.5%)  , which is close to W3C “Silver” at 192 (75.3%)  , and had “Light Gray” at 211 (83%)   and “Dark Gray” at 169 (66%)   counterparts. As a result, the combined CSS 3.0 color list that prevails on the web today produces “Dark Gray”   as a significantly lighter tone than plain “Gray”  , because “Dark Gray” was descended from X11 – for it did not exist in HTML nor CSS level 1[8] – while “Gray” was descended from HTML.