Like HTML, CSS is standardized by the W3 consortium. CSS is an open, independent and freely usable standard. As with HTML, a working group exists for CSS who concerns itself with development of the standard. This is being done in accordance with W3C rules.
CSS comes in different versions. CSS level 1 was released in 1996, and republished with corections in 1999. CSS level 2 appeared in 1998 and built on CSS level 1. CSS level 2 adds support for different output media. CSS 3 is currently in development and will be split up in modules.
The idea behind it was to seperate presentation and content of a web site. The advantage of this approach is that updates and changes to the presentation need only be applied once (to the stylesheet) to affect all pages. Especially large sites can achieve significant time savings. In order to enforce this separation of concerns, the W3C has deprecated the font tag in HTML 4. Newer (X)HTML versions will deprecate other tags in a similar vein.
CSS stylesheets are an immediate addition to HTML. CSS is a language to define format properties of HTML or XML tags. HTML has reached the end of it's shelf life and is not being developed further. XHTML is the new standard instead. XHTML reformulates HTML with XML syntax while at the same time getting rid of presentational markup. XHTML only contains content, presentation is the domain of a CSS stylesheet. This allows for different stylesheets for different output formats such as print, aural or small devices. Search machines on the other hand have an easier job if they are only presented with content markup.
New functionality in CSS2CSS2 retains backward compatibility with CSS1 which means valid CSS1 stylesheets continue to work without changes. CSS2 adds support for different media types. Different stylesheets can target media such as printers, TVs, screen readers and so on. Furthermore, there is the addition of pseudo classes and a few new styling elements for tables, text, colors and positioning.
CSS is supported, to different degrees, by all browsers starting with version 4. Unfortunately, all browsers have implementation bugs, and no browser supports the full CSS2 specification yet. Many browsers allow you to override the provided stylesheet with your own.