The element’s background is displayed and it’s text is moved off-screen so it doesn’t get in the way. Simple and effective. It was often adopted to show graphical titles — that’s rarely necessary now we have webfonts, but you’ll still find it used all over the web.
Link: Goodbye -9999px: A New CSS Image Replacement Technique – SitePoint
The -9999px image replacement technique has been popular for the best part of a decade. To replace a text element with an image, you use the following code: