index.html

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1, minimum-scale=1, maximum-scale=1, user-scalable=no">
    <title>canvas-draw</title>
  </head>
  <body>
    <div id="app"></div>
  </body>
  <script>
    (function(defaultFontSize, defaultScreenWidth) {
      var htmlNode = document.getElementsByTagName('html')[0];
      function resize() {
        var screenWidth = document.body.offsetWidth;
        var fontSize = (screenWidth / defaultScreenWidth) * defaultFontSize;
        htmlNode.style.fontSize = fontSize + 'px';
      }
      document.addEventListener('DOMContentLoaded', function() {
        resize();
      });
      window.addEventListener('resize', resize);
    })(10, 375);
  </script>
</html>