While XHTML and CSS are a powerful combination, there is one small wrench that has continued to plague Web designers: browser support. While it didn't seem to be much of a problem to add extensions willy-nilly, when it comes down to serious, full support of the specifications, no browser has yet been up to the task. However, it's important to note that they've come a long way.
Netscape 6, completely reformed from its extension-madness days, now boasts good CSS support. Too bad its user base is down to less than 1 percent. Firefox 1.5, the Open Source dynamo which rose from the ashes of Netscape's demise (and was even called Phoenix and Firebird early in its history), has excellent CSS support as does Opera 9, whose user base is expanding by leaps and bounds particularly in the handheld and mobile telephone markets. And Internet Explorer, currently the most used browser, has steadily improved its CSS support, although it still has a number of glaring bugs and what sometimes seem like arrogant and obstinate omissions.
All in all, most users use browsers that support CSS either well, or very well. While the number of users on legacy browsers a few years ago might have given folks pause before contemplating a switch to CSS, that number has dwindled below 5% (some say below 2%) and continues to fall. And even many of these are on Internet Explorer 5.5 whose support, though not stellar, was really not that bad.
In short, there's never been a better time to move confidently over to CSS.