rem.js

/* 
*  rem.js 
*  v0.1.1
*  fixed 2015-3-12
*/
(function (win){
  var doc = win.document,
      html = doc.documentElement,
      option = html.getAttribute('data-use-rem');
 
  if( option === null ){
    return;
  }
  
  // defaut baseWidth : 640px；
  var baseWidth = parseInt(option).toString() === 'NaN' ? 640 : parseInt(option);
  var grids = baseWidth / 100;
  var clientWidth = html.clientWidth || 320;

  // set rem first
  html.style.fontSize = clientWidth / grids + 'px';
  
  // create testDom
  var testDom = document.createElement('div');
  var testDomWidth = 0;
  var adjustRatio = 0;
  testDom.style.cssText = 'height:0;width:1rem;';
  doc.body.appendChild(testDom);

  var calcTestDom = function(){
    testDomWidth = testDom.offsetWidth;
    if( testDomWidth !== Math.round(clientWidth / grids) ){
      adjustRatio = clientWidth/grids/testDomWidth;
      var reCalcRem = clientWidth*adjustRatio / grids;
      html.style.fontSize = reCalcRem + 'px';
    } else{
      doc.body.removeChild(testDom);
    }
  };

  // detect if rem calc is working directly
  // if not , recalc and set the rem value
  setTimeout(calcTestDom, 20);

  var reCalc = function(){
    var newCW = html.clientWidth;
    if ( newCW === clientWidth ){
      return;
    }
    clientWidth = newCW;
    html.style.fontSize = newCW*(adjustRatio?adjustRatio:1) / grids + 'px';
    // if( testDom ) setTimeout(calcTestDom, 20);
  };

  // Abort if browser does not support addEventListener
  if (!doc.addEventListener){
    return;
  }

  var resizeEvt = 'orientationchange' in win ? 'orientationchange' : 'resize';

  win.addEventListener(resizeEvt, reCalc, false);
  // detect clientWidth is changed ?
  doc.addEventListener('DOMContentLoaded', reCalc, false);
  
})(window);