I'd be curious to see this backed up. Certainly with a long enough timeline we are not at a maximum of "works only in x", and -- ignoring those beleaguered Opera users still subjected to crappy UA sniffing -- I can't think of the last time I came across a site that blocked me for whatever my choice of browser (I'm a Chrome and Firefox user as well).
If, on the other hand, you're talking about fancy new CSS3 or WebGL (or whatever) demos leaving out some browser due to neglecting to add a particular prefixed property (or whatever), I don't really see that as a problem. Prototype then standardize, right? Yes, not feature detecting and prefixing properly is poor web development practice and yes, they are cutting out part of their audience, but using an experimental feature inherently means that it may break at some point, and almost certainly isn't supported in all browsers.
When 3d CSS transforms land in Firefox soon, there are going to be a whole lot of demos out there that won't work. I would say that's an education and library problem, not a fragmentation one.