/box.html#margin-properties" />
        <meta name="flags" content="ahem" />
        <meta name="assert" content="The 'margin-bottom' property sets a nominal length value in 'ex' units with a plus sign." />
        <style type="text/css">* { margin: 0; padding: 0 }
            div
            {
                font: 20px/1 Ahem;
            }
            #wrapper
            {
                height: 8.5ex;
                border-bottom: 1ex solid red;
            }
            div div
       