Hi webwork, and thanks for the welcome.
Hmmm ... so what has changed? As Ted said, "progress" has been held up, and css2.1 has switched status before, and is only a revision of css2 anyway - so why celebrate given not so much has really changed?
Well, guilt-free coding color:transparent ;)
For me the real significance of the three is that they confirm css is present, active, and current. Regardless of the flaws/deficiencies, people worked hard on this - and that should be acknowledged too. And css2.1 had to be resolved before css3 could be introduced - we see the implications of that in Colour being elevated on the very same day.
More broadly, we can argue the imperfections of the process, technical detail, and political issues. But (IMO) css2.1 at recommendation status emphasise the importance of the basic tools of the craft. I understand the excitement with the visuals available in css3, but the formatting models are the foundation for delivering usable, accessible content to users. Colour reinforces css is responding to technical and other changes, and MathML is a reminder to keep exploring and learning. A base of uniform knowledge, awareness and responsiveness to change and opportunities to expand knowledge are the minimums for any useful occupation - and gettihng all three on one day is worth celebrating.
What would I urge others to adopt/apply/implement? Not intending to be snide, but the recommendations!
Like them, dislike them ... learn them.
The abstract and sections 2.3-2.4 of css2.1 are easy reading and lay the philosophical foundation that puts the rest in context. I expect experienced coders are finding the css2.1 diff file [w3.org] more helpful as they're more concerned with the micro-detail, and it has been colour-coded to visually identify what has changed or been clarified. Of course the Implementation Report [w3.org] gives us a "snapshot" that helps when trying to resolve non-conformance issues.
@Luce, I'm calling "not fair" given font-size-adjust was removed - you have to choose something else ;)
I'm sure you've spotted it in CSS3 Fonts [w3.org] with medium priority, but the working group discussed it on June 11 so perhaps it's being pushed forward - like you I think it has lots of potential. But did any browser ever support it?
That highlights that getting the recs in place is one thing - for change, getting user agents to conform with the candidates is another. We can push user-agent adoption along by user vendor extensions to signal we want those properties/values in css3. But how do we push for useful things (like font-size-adjust) which aren't supported at all?