A good reason not to use the -9999px method is that the browser has to draw a 9999px box for each element that this is applied to. Obviously, that potentially creates quite a performance hit, especially if you're applying it to multiple elements.
Alternative methods include this one (from zeldman.com):
text-indent: 100%; white-space: nowrap; overflow: hidden;...or alternatively (from here ):
height: 0; overflow: hidden; padding-top: 20px;(where 'padding-top' is the height you want the element to be).
I think the first method is neater, since it lets you set height in the normal way, but I've found the second one to work better in IE7
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