[code:css]
/*
* Two divs aligned, split up by a 1em margin
*/
#a {
  width:75%;
  margin-right: 1em;
}
#b {
  width: -moz-calc(25% - 1em);
}
/*
 * Make sure input field won't overlap parent
 */
input {
  padding:2px;
  border:1px solid black;
  display:block;
  width: -moz-calc(100% - 2 * 3px);
}

/*
 * combine different units!
 */

width: -moz-calc(3px + 50%/3 - 3em + 1rem);