title: what is the use of sr-only-focusable/visually-hidden-focusable class in bootstrap?
body: <p>Clearly there is not much documentation or articles about <code>sr-only-focusable</code> (in recent Bootstrap called <code>visually-hidden-focusable</code>). The official documentation here <a href="http://getbootstrap.com/css/#helper-classes-screen-readers" rel="nofollow noreferrer">http://getbootstrap.com/css/#helper-classes-screen-readers</a>, seems not so much understandable. Can anyone help me out in this?</p>