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/* 1. write it out for older browsers */
/* 2. use the vendor prefix for webkit */
/* 3. use the vendor prefix for moz */
/* 4. include the un-prefixed version last */

#foo {
	width: 200px;
	width: -webkit-calc(50% - 100px);
	width: -moz-calc(50% - 100px);
	width: calc(50% - 100px);
}