Make the Colibri CSS pass the CSS 2.1 validation

The reason is to conform to the guidelines:
"Use CSS Level-2.1 of the W3C specification when designing websites."

For example of errors see http://bit.ly/4x9YlM

Questions to answer:

• Do we want to allow some CSS3 rules? Sergiu says: "We should use CSS3 only to enhance visual stuff when without these rules it looks good". Question: How do we ensure that the CSS3 rules we have are only additional visual enhancement and that without them the webapp still works fine?
• What do we do about stuff valid only for FF? For example: "Property -moz-border-radius doesn't exist : 5px". Are there ways to do without them in valid CSS 2.1?