GQ: Why would I want to put information on my web page that is not visible to the typical viewer?

I would want to put information on my web page that isn't visible to the typical viewer because it could be something that changes the color or format of the text/background and so the viewer won't be able to see what things like the font are or the different colors used to make the one presented in the text. Also the metadata on the page is needed to specify what the character set is, the description of the page, the author and more.