ビューポートの互換性の問題に関するコード例

<meta id="viewport" content="
    target-densitydpi=device-dpi, 
    user-scalable=no, 
    width=device-width, 
    initial-scale=0.5" name="viewport" />

var viewPortScale;
var dpr = window.devicePixelRatio;
if(dpr <= 2) {
    viewPortScale = 1 / window.devicePixelRatio;
} else {
    viewPortScale = 0.5
}

document.getElementById(&#39;viewport&#39;).setAttribute(
        &#39;content&#39;, 
        &#39;user-scalable=no, 
         width=device-width, 
        initial-scale=&#39; + viewPortScale);

// alert(viewPortScale);