html5 屏幕自适应代码

var standardDpi,dpi,w,scale;
w = window.screen.width;
if(w>0){
    if(w < 320){
        standardDpi = 120;
    }else if(w < 480){
        standardDpi = 160;
    }else if(w < 640){
        standardDpi = 240;
    }else if(w < 960){
        standardDpi = 320;
    }else if(w < 1280){
        standardDpi = 480;
    }else{
        standardDpi = 640;
    }
}
dpi = 640*standardDpi/w;
dpi = Math.floor(dpi);
scale = 640/w;
document.querySelector("meta[name=viewport]").setAttribute('content','width=640,initial-scale=1.0, maximum-scale=3.0, minimum-scale=1.0,target-densitydpi='+dpi+', user-scalable=0');
if ("-ms-user-select" in document.documentElement.style && navigator.userAgent.match(/IEMobile\/10\.0/))
{
    var msViewportStyle = document.createElement("style");
    msViewportStyle.appendChild(
        document.createTextNode("@-ms-viewport{width:auto!important}")
    );
    document.getElementsByTagName("head")[0].appendChild(msViewportStyle);
}