[bellerocky]

I haven't worked at a place that has cared about IE since 2009. Besides old versions of IE Polymer works everywhere. Worrying about being backwards compatible with old browsers is a thing of the past with constant updates to Chrome and Firefox. If you're developing a mobile site you can't use any of the newest things until Safari updates (about once a year, seriously) but you should be otherwise ok.

[stereosteve]

I work at a site where about 20% of traffic comes from IE8. Many are users on corporate machines with no alternatives. I really appreciate that React runs on older browsers and is decently performant without a huge payload. All this with a well designed API and a nice development experience... I think it's hard to beat!

Anyway the more traffic your site gets, the harder it becomes to ignore certain cohorts.

[morkbot]

But Polymer depends, for example, on Shadow DOM, which is currently only supported by Chrome, Opera (Chrome-based) and Android browser - not even by Firefox yet (http://caniuse.com/#search=shadow%20dom). There are pollyfills but they're too slow to treat them as a viable solution.