It's a non-standard property on an HTML element which is only supported by IE7 and lower ( IE8 compatability mode too ), which if triggered, causes the element to be rendered in a certain way ( which can be unexpected, random, can be a godsend or can be hell ).
Classic example is giving layout to an element so it can clear floats.
#wrapper { zoom:1; }
The element will now contain floats. Any of these properties and value other than auto/normal will trigger the layout property.
* display: inline-block
* height: (any value except auto)
* float: (left or right)
* position: absolute
* width: (any value except auto)
* writing-mode: tb-rl
* zoom: (any value except normal)
Please have a thorough read @ http://reference.sitepoint.com/css/haslayout
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