The hexadecimal notation
#RRGGBB for RGB colour triples has been popularised by HTML and is commonly associated with it, even though nowadays it is not usually used in HTML directly, but rather through CSS.
I noticed that the same notation is used in the X pixmap format (XPM), which to my knowledge was created in 1989, thus predating HTML (1993).
Where does this notation come from? How did it end up in HTML?