I believe that we're living on a time that Microsoft has abandon ie6 to ie10, Google stopped supporting Chrome browser on old machines with old OS and it is time to start developing for just new devices and updated browsers and charging a lot extra for doing stuff for old browsers and stuff.
Now, going back to your question I believe that if you're doing some stuff on the front end that won't be supported on old browsers but does not affect the purpose of the site or webapp then just do a css condition for old browsers that show something else for those browsers and add an alert that the user should update or change browser. But again if everyone is doing it (big companies and the ones that developed the browser) then stop doing support for old browsers. I believe that the ones that truly needs support in some cases and countries are banks that has all their software and stuff running on very primitive computers, browsers and OS but if you ask me they should update for security reasons.