Unsupported background/border colors such as LightGray, or DarkOrange

Hello everyone!

 

It seems that colors such as LightGray, or DarkOragne, are not supported by
the W3C CSS validator (see for example the validation results of
http://www.w3schools.com/tags/ref_color_tryit.asp?color=LightGray). How
come? It seems to be a valid value for the color property, isn't it?

 

Thanks!