The beauty about working on a site that is specifically targeted for one the most popular mobile devices around (aka the iPhone) is that the vendor with the least CSS support (*cough* IE *cough*) doesn’t exist on it (purely because of it’s inadequate CSS support in this case).
Depending on the browser matrix you have to work with and the platforms you’re coding for, utilizing Level 3 (and some Level 2.1 in IE’s case) selectors simply isn’t realistic, although properties on the other hand can be used to progressively enhance an elements appearance in this particular context.
But what if a site was created to serve one specific handheld device in which Safari (along with it’s sufficient CSS3 support) was the default browser? Well, this is what Facebook has done with it’s iPhone-specifc UI. By peaking into their stylesheet you’ll notice that they’re implementing a number of Level 3 selectors, properties and property value additions. They include the:-