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."

In other words, "modern browsers" might as well be mythical creatures :)

Roughly, self updating html5 compliant browser.

Why self-updating? Is my version of Firefox non-"modern" simply because I install and update it through my operating system's package management system?

It is if you updated it recently.

In general as regards SVG [and canvas in this case], not IE <=8 or Android <= 2.4, and some caveats (eg will need careful testing, as some JS/DOM APIs or rendering optimisations may not be present) with earlier versions of FF/Chrome/Safari & IE9/10

what about in a year? i bet you would list different browsers and versions. my point is, "modern browser" doesn't mean anything and it change as time passes

True, it should specify where support drops off really, it's not as if there aren't tools to automate the JS testing part of it if the time isn't there to thoroughly manually check. I've think maybe spent too much time assessing JS libraries & frontend stuff; after a while when you see 'modern browser' you mentally tick off where it's likely to be functional and accept/discount the guessed tradeoffs (maybe a bad habit)