What is wrong with designing browser specific websites?
Designing websites to work only in specific browsers isn't a percentage game. It is, however, a self-fulfilling argument that makes the web-developer worse off for going the "one-browser" route.
The "one-browser" mentality is so unrealistic, its the assumption that everyone in the world is exactly the same as you, therefore they all run the same hardware and software for the same purpose, with the same interests. It is also a total inability to deal with the past and future as well as the present.
The lesson of history is that browser usage can change dramatically in a matter of months - Netscape's browser took 70% of the market in 1996. The way to deal with that knowledge is to ensure that sites can cope with such changes without having to be rebuilt
Authoring multiple sites for multiple devices
By entirely relying on software and the assumption that web design is simply a different form of DTP you can create something that looks nice in a very limited range of visual browsing situations and which falls apart catastrophically in most others.
By learning what you are actually doing you can make a site that works effectively pretty much all of the time, looks and sounds good in the vast majority of browsing situations...and which can easily be adapted to changes in technology or fashion as and when required