This web page has an alternative stylesheet named green
assigned to it. Use your web browsers stylesheets-switching option to format the page with that stylesheet.
Now this page and see what happens...
This page neither uses cookies, or a scripting language to retain the information that you switched to another stylesheet. That's why the page background will now be white again.
Couldn't it be a feature of the web browser to remember the stylesheet switch? The browser has all the means to do that, i.e the style sheet, it's name and the URL of the current page and the referer (to check if we are still browsing the same site). So one doesn't have to use cookies when making a web site just to remember the stylesheet choice.