In Dynamics NAV 2013 R2 and 2015 there is a new format for control add-ins. This control add-in is actually done in HTML/Javascript so it will work in both clients.
When running in the Role Tailored Client, a WebBrowser Controls is created for the control, one of the main challenges, is that the WebBrowser Control insist on being a Internet Explorer 7 browser, witch reduces the level of JavaScript that can be used and excludes all HTML5 based controls.