通过改变viewport 实现网站自适应

var phoneWidth = parseInt(window.screen.width);
var phoneScale = phoneWidth/640;
var userAgent = navigator.userAgent;
var index = userAgent.indexOf("Android");
if(index >= 0){
var androidVersion = parseFloat(userAgent.slice(index+8));
if(androidVersion>2.3){
$("head").eq(0).append("<meta name='viewport' content='width=640, initial-scale = "+phoneScale+", minimum-scale = "+phoneScale+", maximum-scale = "+phoneScale+",user-scalable=no, target-densitydpi=device-dpi'>");
}else{
$("head").eq(0).append("<meta name='viewport' content='width=640, target-densitydpi=device-dpi'>");
}

}else{
$("head").eq(0).append("<meta name='viewport' content='width=640, initial-scale = "+phoneScale+", minimum-scale = "+phoneScale+", maximum-scale = "+phoneScale+",user-scalable=no, target-densitydpi=device-dpi'>");
}

(function(a, d) {
var b = a.documentElement,
e = "orientationchange" in window ? "orientationchange" : "resize",
c = function() {
var a = b.clientWidth;
a && (b.style.fontSize = Math.min(a, 640) / 320 * 100 + "px")
};
css = document.createElement('style');
css.type="text/css";
css.innerHTML = "html {font-size:"+Math.min(b.clientWidth, 640) / 320 * 100+"px}";
a.getElementsByTagName('head')[0].appendChild(css);
a.addEventListener && (d.addEventListener(e, c, !1), a.addEventListener("DOMContentLoaded", c, !1));
})(document, window);