I guess it would be something along the lines of this quote from http://farukat.es/journal/2011/02/528-modern-browser: "A modern browser is any browser that: successfully renders a site that you just built using web standards, testing only in your browser of choice along the way, with all the essentials functioning well; without you having written any browser-specific hacks, forks or workarounds; and shows great performance as you navigate it."