Unsupported character set

I was trying to check to see if my page was valid, and I was surprised by
the error I got.

> A fatal error occurred when attempting to transliterate the document
> charset. Either we do not support this character encoding yet, or you have
> specified a non-existent character encoding (typically a misspelling such as
> "iso8859-1" for "iso-8859-1").
> 
> The detected charset was "x-mac-roman".
> 
> The error was "x-mac-roman undefined; replace by macintosh".

The part I really don¹t understand is the last part, replace what, by
macintosh?

Also, I would think that this would be supported, but obviously not.  If you
know a way to change what the character set is in the HTML Headers, please
let me know.