Web Standards Programmer's Reference: HTML, CSS, JavaScript®, Perl, Python®, and PHP

12.3. CSS Levels 1, 2, and 3

There are three levels of CSS—two levels are actual specifications while the third level is in recommendation status. The main differences between the three levels are as follows:

• CSS1 defines basic style functionality, with limited font and limited positioning support.

• CSS2 adds aural properties, paged media, and better font and positioning support. Many other properties have been refined as well.

• CSS3 adds presentation-style properties, allowing you to effectively build presentations from Web documents (similar to Microsoft PowerPoint presentations).

You don't have to specify the level of CSS you are using. However, you should be conscientious about what user agents will be accessing your site. Most modern browsers support CSS, but the level of support varies dramatically between user agents. It's always best to test your implementation on target user agents before widely deploying your documents.

When using styles, it is important to keep in mind that not all style properties are well supported by all user agents. This book attempts to point out major inconsistencies and differences in the most popular user agents, but the playing field is always changing. One invaluable reference for style compatibility is Brian Wilson's excellent resources at http://www.blooberry.com/indexdot/index.html.