Meta tag for designing on a 640px mobile screen, targeting the font to then match it unless it's an ipad.

mobile.html

<meta id="viewport" name="viewport" content="width=320, intial-scale=0.5, maximum-scale=.5, minimum-scale=.5, user-scalable=no"/>
<script>
  (function(doc) {
    if (navigator.userAgent.match(/iPad/i)) {
      doc.getElementById("viewport").setAttribute("content", "initial-scale=1, maximum-scale=1, minimum-scale=1, user-scalable=no");
    }
  }(document));
</script>