I've been designing Web pages for over a decade and didn't realize there were this many named colors.

These colors were first named by Netscape back in the day. All browsers since then have adopted them for the sake of compatibility.

There is also a larger set of *fake* color names, usually including colors with numbers at the end, such as "indianred1". The only browser that supports those colors is Opera.

Regardless, it's still usually best to specify colors as hex.