<!DOCTYPE html>
<html>
  <body style="zoom: 1.1;">
    <div id="test" style="height: 100px;">
      <div id="red" style="width: 212px; height: 100px; background:red; float: left;"></div>
      <div id="blue" style="height: 100px; background: blue; float: left;"></div>
    </div>
    <script>
      var testContainer = document.getElementById('test');
        var w = document.body.getBoundingClientRect().width;
        w = 913.0908660888672;
        testContainer.style.width = w + 'px';
        testContainer.children[1].style.width = (w - testContainer.children[0].getBoundingClientRect().width) + 'px';
    </script>
  </body>
</html>