title: css element height minus height of a element with changing height
body: <p>aye folks!</p>
  
  <p>is it possible to have dynamic height calculation with css3? i couldn't find anything.. but maybe i'm just using the wrong terms for my search.</p>
  
  <p>the problem: i have a site where i want to include an iframe with 100% height minus the height of a nav-element. unfortunately the nav-element isn't always the same height (44px on one device 36px on another.. and so on)</p>
  
  <p>is there a way to calculate that? or do i need to fix the height of the nav-element to get this to work properly?</p>