These CSS snippets are all intended to be functional on as many browsers and platforms as possible, but do not function with older browsers (such as Netscape 4) which do not support most of CSS1.
These pages often have examples where the same HTML snippet is used but with different CSS. The CSS files shown have different class names, even though the HTML snippet's source isn't shown again, so the different examples can be repeated on the same page.
Effort has gone to be sure the CSS used and its particular effect are associated in the text. Using your browser's View Source option will show you exactly which CSS affects which HTML block.