Cascading Style Sheets was developed by a World Wide Web Consortium team headed by Bert Bos and Håkon Lie. The intent of the project was to create a styling language that could be integrated with HTML and XHTML to complement its structuring capabilities with styling rules.
CSS has been in the planning stages as long as HTML. But when HTML first came out, the technology did not exist to make use of CSS. CSS was not implemented as a standard until after computer monitors allowed for full multimedia display, instead of just spiffied up text.
By the time CSS was released as a standard the browser companies had altered HTML to include many styling commands in order to make HTML documents more attractive. This means that although the idea for CSS has been around as long as HTML, it is playing catch up with HTML technology. It is not so much as new set of standards standard as a pulling back in line of markup languages with the original conception of the standards.
The first version of CSS, CSS1, was released in 1996 and includes basic styling functions such as font, color, and background images. Most current browsers fully support CSS1.
CSS2 came out in 1998, and it added some high end features to CSS. CSS2 allows for positioning of elements on the page for page layout, provides support for downloadable fonts, and allows pages to be formatted for printing.
By providing a means to avoid the use of tables in page layout, and by allowing pages to be formatted entirely differently for paginated (printed) layout versus continuous (on-screen) layout, CSS2 has become a language that has gone beyond anything one could ever do with HTML.
Currently, no one fully supports CSS2, although Opera 6 and Netscape 6 both have almost total support. Exploror 6 has extensive support. Netscape 4.x has almost no CSS2 support.
A browser or other user agent needs to be as close as possible to fully CSS2 compliant in order to make use of CSS with XML.
CSS3 is still in development, but will add support for additional presentation media to the mix, such as aural (audio) styling. CSS3 will also involve a restructuring of CSS into modules that support different styling elements. This will not affect older style sheets, it will merely provide a better way of structuring new ones.
Most of what is going on in the development of CSS3 will not affect the way in which you use CSS to format documents for display on a computer screen.
XSL, which is short for Extensible Stylesheet Language, is a styling language that has some relationship with CSS, but also goes far beyond anything CSS can do. XSL is specifically designed for styling XML documents, and is close to a full programming language. It allows the programmer to do such things as convert XML into different markup languages (such as XHTML) for presentation in different media or for use in different applications.