Agree with @Robert, not a fan of polyfills, they're slow and often buggy. Unless the market I'm entering requires support of older browsers, I simply don't cater for them. If somebody is using an ancient browser, chances are they're also not going to spend.
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.
With Modernizr, you can provide what's needed to legacy browsers. This is used widely in the Telemedicine industry.
Robert van der Elst
Front End Designer
Most people would say polyfills, but I'm very hesitant to use them.
I tend to design/code for the newest browsers and let the older browsers be. If older browsers don't support it (gradients, border-radius etc.) it won't show and that's just fine I think.
You can't code a site and expect it to work perfectly on all browsers that are used today (including IE7 and IE8 ;) ). It's all moving so fast that it's impossible to cater for all of them. And the Chrome of today will be considered old next year or the year after.
As long as the HTML/CSS/JS won't break in the older browsers than it's fine. It should remain functional and usable, but if doesn't look exactly 100% the same as the design, then so be it.
Just my view of course :)