Sergio is exactly right, if the parent inherits the index, the child can be negative and hide. At least this is true in my tests. Also note that for a child, relative / absolute / fixed positioning must be used.
http://jsfiddle.net/6xT45/
#parent { position: absolute; width: 100px; height: 100px; background: blue; } #child { position: absolute; z-index: -1; width: 50px; height: 50px; top: 25px; left: 25px; background: red; }
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