Why does CSS3 use two names for the same color: aqua = cyan, magenta = fuchsia?
Why does CSS3 duplicate
#00ffff = 0,255,255 and
#FF00FF = (255, 0, 255), spotted by this user?
This duplication confuses, as each is listed separately on the aforementioned CSS3 link.
There are a few clashing/duplicate colors listed in CSS3 because the current web colors list maintained by W3C was descended from the X11 color names file.
The X Window System contained a text file that had RGB values that mapped to certain colors. This text file was shipped with all X11 installations.
Mosaic and Netscape Navigator both used the X11 color names as the basis of their colors.
Once the HTML color list was defined, instead of deprecating the duplicate and clashing colors, they simply adopted them into the list - presumably to prevent breaking older websites/browsers that used X11 colors and because X11 had defined them first.
That being said, if you use a clashing color name on a site today, it would display the W3C color instead of the X11 color.