动态设置font-size

针对以rem为单位的项目，计算html的font-size ,

// 计算html的font-size  
(function(){ 
    function resizeBaseFontSize(){ 
      var rootHtml = document.documentElement,
          deviceWidth = rootHtml.clientWidth; 
      if(deviceWidth > 640){ 
         deviceWidth = 640;  
      }  
      rootHtml.style.fontSize = deviceWidth / 7.5 + "px";  
   }  
   resizeBaseFontSize();  
   window.addEventListener("resize", resizeBaseFontSize, false);  
   window.addEventListener("orientationchange", resizeBaseFontSize, false);  
})();