The other responder is accurate. CSS is not nearly to the place where XSL FO is.
Now -- I am part of RenderX as a company, I own the Cloudformatter environment and co-developed such things as Css-to-pdf as well as Nimbus.
As an experienced standards person, my estimate is CSS is 8+ years away. But who knows? CSS moves in many ways and pagination and formatting is but one (minor) way it moves. And HTML(5) moves so far away from XML, trying to keep track of the decisions made is, well scary.
The real question you should ask is what are you looking to do? Formatting documents with XSL FO has been in production (since about the year 2002) and doing so for many millions of very complex pages each and ever day (like your phone bill or 401K statement). Formatting pages from HTML is in production for its own purpose, but not those kind of things. No way.
XSL FO will be used for the next 10+ years to do the tasks it perfectly accomplishes today.
The concept of pages and keeps and many other things that make complete sense to printed formats are foreign or only being considered in CSS. Not to mention that most if not all these types of software packages produce only PDF ... as if that is the only format in the world to print. Well, there are many more not the least of which is AFP and others like PPML and Postscript.
There are also hybrid solutions we have done, like http://www.cloudformatter.com/Nimbus. This is an XSL FO designer but uses HTML and CSS for you to specify the design. You can add all the logic you want. You get XSL for XSL FO (it converts the flattened CSS to XSL constructs for FO and doesn;t try to use CSS for formatting).
So. Modern? XSL FO is modern, really. It is a rich standard that accomplishes the task it is designed to do. HTML+CSS is made to do something else and partially be flexed to accomplish another task. Should you wait or? That is up to you and what you think you will get and what the risk is.
