refer: http://css-discuss.incutio.com/?page=ScreenreaderVisibility
display:none: will not be seen nor heard. *
visibility: hidden: will not be seen nor heard. *
text-indent: 9999: will not be seen but it will be heard.
- Most of the screen reader will not ‘speak’ display:none and visibility: hidden , but there are few screen readers like pwWebSpeak and HtReader which will read even these too.