Responsive Design using w3.css
The elements of the W3.css Framework are responsive by design without having to take additional steps to make your design compatable on all devices.
This page discusses the use of a W3.css template and the fact that it uses a "hidden" control. The issue is discussed in more detail on my page on the Device Dependent Handler validation error.
For a more general discussion regarding W3.css and responsive design
A hidden Control
Button below will be hidden by the w3-hide-large class on large screens.
This button is used on my new Friends/PC template and if the same thing that happens when the page is checked with the Wave Accessibilty tool then I may have a clue in how to fix it.

As seen with my Friends/PC template the Wave Accessibilty tool reports a "Device Dependent Event Handler".