移动端rem设置

<script type="text/javascript">
        (function(){
            var resizeEvent =  'orientationchange' in window ? 'orientationchange' :'resize';
            var rescale = function(){
                document.documentElement.style.fontSize = document.documentElement.clientWidth / 640 * 20 +'px';
            }
            window.addEventListener(resizeEvent,rescale,false);
            document.addEventListener('DOMContentLoaded',rescale,false);
        })();

        </script>