The colours used on the web used to be a simpler list, but when the X11 colours were merged into the list there were some conflicts, which is why the default CSS grey doesn't lie between light and dark grey.

I included a line of pale colours because I always wonder how often colours like these actually get used. I can't say I've ever had a use for these, especially when colour selecting a custom hex code is so easy.

I have no idea why the sea greens are like that. It seems like an X11-CSS merge issue but I don't have a good source for it (not that I searched super hard).

Indigo is purple on the web because the references for the X11 colours were Crayola crayons and children's paints. This also had the side effect of there being a bunch of strange CSS names like peachpuff.