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'>");
    }