The Internet Explorer 8 Release Candidate is the last major IE8 testing milestone. It indicates that we believe that IE8 is implementation complete for CSS 2.1. We also believe IE8 RC1 has the most complete implementation of the CSS 2.1 specification in the industry.
The only way to know if a browser has correctly implemented a specification is to develop a comprehensive set of tests for the specification. These can be used to determine both the support for a specific part of the spec and the behavior of a specific browser. Web developers can also use these test pages as examples of how to combine various layout properties and elements in their pages and know that their page will interoperate well across all the browsers that pass those tests.