title: Set line-height as a percentage relative to the parent element
body: <p>I have a responsive element where it's width and height will both scale. Inside this I have some text which I want to center vertically.</p>
  
  <p>How can I set the text's <code>line-height</code> to be the same as it's parent if I don't know the parent's height?</p>
  
  <p><code>line-height: 100%</code> is relative to the font's regular height so this doesn't help...</p>