HTML5 自适应rem布局

(function(doc, win) {
var docEl = doc.documentElement,
resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
recalc = function() {
var clientWidth = docEl.clientWidth;
if (!clientWidth) return;
if(clientWidth > 640){
clientWidth = 640;
}
docEl.style.fontSize = 20 * (clientWidth / 640) + 'px';

if(window.orientation){
if(window.orientation == 90 || window.orientation == -90){
docEl.style.fontSize = 12 * (clientWidth / 640) + 'px';
}
}

};
recalc();
if (!doc.addEventListener) return;
win.addEventListener(resizeEvt, recalc, false);
doc.addEventListener('DOMContentLoaded', recalc, false);

//viewport问题
/*var devicePixelRatio = document.devicePixelRatio;
if(devicePixelRatio == 2){
document.querySelector('meta[name="viewport"]').setAttribute('content','initial-scale=0.5, maximum-scale=0.5, minimum-scale=0.5, user-scalable=no');
}else if(devicePixelRatio == 3){
document.querySelector('meta[name="viewport"]').setAttribute('content','initial-scale=0.3333333333333333, maximum-scale=0.3333333333333333, minimum-scale=0.3333333333333333, user-scalable=no');
}*/

var wAlert = window.alert;
window.alert = function(message) {
try {
var iframe = document.createElement("IFRAME");
iframe.style.display = "none";
iframe.setAttribute("src", 'data:text/plain,');
document.documentElement.appendChild(iframe);
var alertFrame = window.frames[0];
var iwindow = alertFrame.window;
if (iwindow == undefined) {
iwindow = alertFrame.contentWindow;
}
iwindow.alert(message);
iframe.parentNode.removeChild(iframe);
} catch (exc) {
return wAlert(message);
}
}
var wConfirm = window.confirm;
window.confirm = function(message) {
try {
var iframe = document.createElement("IFRAME");
iframe.style.display = "none";
iframe.setAttribute("src", 'data:text/plain,');
document.documentElement.appendChild(iframe);
var alertFrame = window.frames[0];
var iwindow = alertFrame.window;
if (iwindow == undefined) {
iwindow = alertFrame.contentWindow;
}
var rr = iwindow.confirm(message);
iframe.parentNode.removeChild(iframe);
return rr;
} catch (exc) {
return wConfirm(message);
}
}

})(document, window);