Actually, a new technique came out recently. This article will answer your questions: http://www.zeldman.com/2012/03/01/replacing-the-9999px-hack-new-image-replacement
.hide-text {
text-indent: 100%;
white-space: nowrap;
overflow: hidden;
}
It is accessible, an has better performance than -99999px.
Update: As @deathlock mentions in the comment area, the author of the fix above (Scott Kellum), has suggested using a transparent font: http://scottkellum.com/2013/10/25/the-new-kellum-method.html.