It is no secret that modern browsers get updates all the time to fix security issues and improve performance. These updates are free, so there is no reason not to run the latest version of your favorite browser. The problem is many of us forget to upgrade our browser. If your site does not run well on very old browsers, you can use the WP BrowserUpdate to remind your visitors to update their browser.
The WP BrowserUpdate is very simple to use. Just install it, and you will be able to remind your visitors to update their browser. Of course, you can refer to Browser-update.org to find out how you can further customize this. This is a free plugin.