CSS2.1 & CSS3
CSS2.1 is built off of CSS2 and is meant to replace it in documents and functionality. Some parts going from CSS2 to CSS2.1 have been kept the same, some have been changed, and some have been removed completely. Anytime code that is currently being used is removed from the standard, there will be someone who will have to change their code. The best way to do things is to continue to allow outdated elements, but have new alternatives that replace them so people can begin to implement those instead, while their old pages will still continue to work.
An interesting thing that has been added is inheritance. Inline elements will inherit properties of block-line in which they reside. Many layout calculations (such as background position) can also now be inherited. Another interesting thing is that anything with a color attribute can now be set to transparent and will show through whatever is behind it."Position" can now be applied to any element to help keep layout the way it is wanted. In CSS2.1, some new features for style sheets can be added, including Aural style sheets which will give values that define speech output. This will be an incredible feature that will help web pages become more accessible to many more people.
CSS3 is more of a look/feel type markup that will allow for web pages to implement a wider variety of styles to make a page look good. Some examples are borders that can fade from one color to another, shadows underneath text, multiple backgrounds, outlining, and resizing. Many of these functions can be helpful, but most are not necessary, and just add to how a page presents itself. The additional attribute selectors are allowed to match substrings when looking for what CSS style to implement. It can be used to make it so that each element doesn't need to have the exact same id to be able to call its attributes from the style sheet, but instead, many different ones with different values in their attribute can share their style.
Currently, Internet Explorer 4-7, Firefox, Safari, Netscape, and Opera all implement the use of CSS1 and CSS2. Use of a newly implemented version, whether CSS2.1 or CSS3 would take time to establish and work throughout the world wide web. Many people object to adding new standards at this time, and some are beginning to doubt W3C's capability of continuing to keep things up-to-date and are wondering if there should be another entity in charge of determining CSS styles that should be generally used.