Remove certain DIVs when viewing on a mobile device.

Responsive CSS

@media only screen
and (min-device-width : 320px)
and (max-device-width : 480px) {
 .qrcode widget {
      display: none
}

@media screen and (min-width: 0px) and (max-width: 400px) {
  #my-content { display: block; }  /* show it on small screens */
}

@media screen and (min-width: 401px) and (max-width: 1024px) {
  #my-content { display: none; }   /* hide it elsewhere */
}