I am working on a react application and customers want it to show a special message if user's old browser does not support (e.g. IE 9).
So long I tried to detect some "popular" old browsers, using react-device-detect package. 
src/index.js
import { browserName, browserVersion } from "react-device-detect";
const render = Component => {
  if (browserName === "IE" && browserVersion < 10) {
    ReactDOM.render(<UnsupportedBrowser />, document.getElementById("root"));
  } else {
    ReactDOM.render(
      <AppContainer>
        <Component store={store} history={history} />
      </AppContainer>,
      document.getElementById("root")
    );
  }
};
And putting conditional comments:
public/index.html
<!--[if lte IE 9]>
  Please upgrade your browser
<![endif]-->
But I have a suspicion, that there are better ways of doing it, which I could not find searching the web.
 
     
     
     
     
     
    