fadeyev, your first example is a little misleading. Only Safari 3-4 required the border-radius syntax to be declared individually. So really this is how it goes:

     /* these two lines for saf 3-4 support */
       -webkit-border-top-left-radius: 5px; 
      -webkit-border-top-right-radius: 5px;

    /* all you need for Saf5, Chrome, Op, FF1+, IE9 */
     -moz-border-radius: 5px 5px 0 0;
          border-radius: 5px 5px 0 0; 

That said, I like the project. Just clarifying this. :)

EDIT: given the spec'd shorthand syntax, your shorthand for unique corners seems to conflict `(5px, 0, 0, 5px)`. I'd suggest changing that.