I just came across this article in which Drupal 7 provides two classes in the theme layer to help design for screen readers. One hides from sighted users but still allows a screen reader to access the content. The other shows the content when a user tabs over to it.
Two separate classes have been defined, one to be used for elements that cannot receive focus, and a second for elements that can receive focus. When a focusable element is styled with the .element-invisible.element-focusable class it will appear visually when it receives focus. This is to help keyboard users to not lose the system focus into an invisible black hole.