lLR element inside an horizontal container computes properly its percentage margins against the container's inline size (the container's width in this test), and the element is sized accordingly subtracting those margins from the available size.">
<link rel="stylesheet" type="text/css" href="/fonts/ahem.css" />
<style>
.container {
  background: cyan;
  border: solid thick;
  height: 200px;
  width: 500px;
}
.element {
  background: magenta;
  font: 25px/1 Ahem;
  margin: 10%; /* This should be computed aga