element-invisible class doesn't fully hide elements

Over in #718922: system class: .element-invisible does not work with VoiceOver on OS X Snow Leopard and #766444: Add !important to CSS attributes of .element-invisible in system-behavior.css, we've found some issues with the original D7 implementation of the element-invisible class.

The most recent patch in #718922 changes:

.element-invisible {
  height: 0;
  overflow: hidden;
  position: absolute;
}

to:

.element-invisible {
  position: absolute !important;
  clip: rect(1px 1px 1px 1px); /* IE6, IE7 */
  clip: rect(1px, 1px, 1px, 1px);
}

Zen should use the same.