Using ARIA (Accessible Rich Internet Applications) markup, it is possible to enhance WordPress' accessibility to indicate the "required" state of certain form fields. This is currently indicated visually through styling, and through adding an abbr with title "required" and a * as plain text. But using ARIA markup, screen readers that support this can do more with explicit state "required" for the form field accessibles, for example play a certain sound or the like.
ARIA is unintrusive: If a browser is used such as IE 7 or Safari, ARIA markup won't interfere, it will simply be ignored. Browsers however, that support the WAI-ARIA markup already, will automatically use the extra information, and as more browsers become available, they'll pick it up automatically as well.