For CSS3 is fully backward-compatible, so you will not have to change the existing design. The browser will always support CSS2.
CSS3 is split into "modules." Former standard has been split into small pieces, also added new ones.
Some of the most important CSS3 modules are as follows:
- Box Model
- Background and border
- Text Effects
- 2D / 3D conversion
- Multi-column layout
- User Interface
W3C's CSS3 specification is still under development.
However, many of the new CSS3 properties have been used in modern browsers.