CSS3 is short for CSS (Cascading Style Sheets) level 3 and is the newest changes to CSS.
CSS Level 3 (CSS3) builds on CSS Level 2 module by module, using the CSS2.1 specification as its core. Each module adds functionality and/or replaces part of the CSS2.1 specification. The new CSS modules should not contradict the CSS2.1 specification: only that they will add functionality and refine definitions. As each module is completed, it will be plugged in to the existing system of CSS2.1 plus previously-completed modules.
The following modules are or have been specified: Media Queries Level 3, CSS Namespaces, Selectors Level 3, CSS Color Level 3
ePub 3 will use some of the modules from CSS3 and add proprietary extensions as needed. It will fully embrace the underlying CSS level 2.1 with only minor exceptions where they have opted for HTML5 methods of doing a similar thing.
Firefox has the custom -moz-font-feature-settings CSS property. If kern=1 then kerning will work if you have a nice OTF font chosen. Firefox also has CSS controlled hyphens using moz-hyphens