Currently, we've implemented element-invisible and element-focusable as mixins. This is slightly inefficient because we have to apply all 6 properties each time the mixin is used.
With the release of Sass 3.2 we know have the ability to use %placeholder selectors that can be @extend-ed. That means we create one %element-invisible ruleset and any time we use
@extend %element-invisisble; the selector is copied up to the placeholder's ruleset. (Rather then the other way around.)