According to Microsoft, "E8 RC1 has the most complete implementation of the CSS 2.1 specification in the industry".
We have heard big promises from Microsoft in the past that they have not delivered upon, but recently they have found incentives to play nice and actually follow through on their promises.
If this really signals action rather than mere words, congratulations to Microsoft!
However, while these words from Microsofts are promising, there may be some problems as well. Daniel Glazman comments that some work still remains before Microsoft can claim the CSS 2.1 crown:
Full CSS 2.1 complete support is cool but it must be said that "full CSS 2.1 support" does not mean anything yet. CSS 2.1 is not yet a W3C Recommendation and it does not have an approved full Test Suite yet. Microsoft has submitted a rough 7,000 CSS 2.1 tests to the CSS WG but these tests need review before becoming official.
In the comments, James Hopkins points out some problems as well:
I'd argue that many of those 7,000 test cases are surfeit and unneccessary. For example, they have around 50 test cases specifically testing an absolutely positioned element that is offset to the left, and some are actually invalid.
If Microsoft's claims are based on invalid tests, one can hope that this is merely an error on their part. Normally one would not attribute to malice what could equally well be attributed to mistakes, but knowing Microsoft's history, we should keep scrutinizing their every move, and every claim. "Errors" have historically turned out to be conscious decisions on Microsoft's part to undermine open standards and competition in the market.