This section is an introduction to styles sheets using the Cascading Style Sheet 1 (CSS1) language. The references at the end of this section will refer you to an external web site where you can find the complete specifications for CSS1 and the newer, but more complicated, CSS2 languages. If you prefer a reference book, try "Cascading Style Sheets: Designing for the Web", by Håkon Lie and Bert Bos (Addison Wesley). The authors of this book designed the CSS language and wrote the specifications.
CSS1 is not HTML. It is a language which describes properties of markup elements.
Internet Explorer 4.0 has a fairly complete implementation of the core parts of CSS1, while Netscape Navigator 4.0 has only a partial implementation. Both Netscape and Microsoft have expressed a commitment to support CSS. Netscape has implemented style sheets via Javascript - so that must be enabled for style sheets to work on that browser (that is also why not everything works).
Another external reference at the end of this section will take you to sites that show the degree of implementation in the various browsers. The W3C also makes available a set of Core Style Sheets that you may include in your documents freely. These style sheets are encoded to detect the type of browser being used to view a web page and generate appropriate styles based on the level of implementation.