It certainly can make business sense to use technologies that aren't yet supported by 100% of browsers in the wild. • Websites that are mobile-only • Things t

It certainly can make business sense to use technologies that aren't yet supported by 100% of browsers in the wild. • Websites that are mobile-only • Things that aren't websites but use HTML5 for UI (e.g. many native mobile and desktop apps do this, browser extensions, 'installable web apps' for Chrome etc.) • Websites for whom the IE audience is such a minority that it's worth either (a) blocking them out and asking them to upgrade, or (b) maintaining a minimal-effort fallback stylesheet for IE users so they can at least access a low-fi version of the site – in return for the gains in performance and UI quality that the vast majority of users get, and for incalculable benefits in developer happiness and productivity. I do front-end for a newspaper website and I have to offer some support right down to IE7, but it's a realistic guideline (some support – content basically visible, with warnings displayed to those users suggesting they upgrade). If we were to restrict ourselves to browser features supported by 100% of our users, this would be a lowest-common-denominator approach led by the most low-tech 5% of our users, which would give us a big competitive disadvantage. I use new browser features all the time, and fallbacks for IE users. I haven't used Flexbox yet, but I wouldn't hold back if it meant we could implement something good for a large proportion of our users (especially since that proportion is likely to be made up of the users who are responsible for most of our revenue).