title: What is the relationship between target-densitydpi and window.innerWidth with Android H5
body: <p>My genymotion emulator is Google Nexus7 -4.1.1 API16- 800*1280 .
  When I use webview to load HTML5 on Android, I was confused with the relationship between target-densitydpi and window.innerWidth.</p>
  
  <pre><code>&lt;meta name="viewport" content="width=device-width,**target-densitydpi=device-dpi**,initial-scale=1.0,minimum=1.0,maximum-scale=1.0,user-scalable=no"&gt;
  
  $(function(){alert(window.innerWidth);   //get the viewport width
            alert(window.devicePixelRatio);})  //  always is 1.3312499523162842  no matter what target-densitydpi is
  </code></pre>
  
  <p>I do some tests:</p>
  
  <pre><code>                                   window.innerWidth
  </code></pre>
  
  <p><code>**target-densitydpi=device-dpi:**         800</code></p>
  
  <p><code>**target-densitydpi=low-dpi:**            452</code></p>
  
  <p><code>**target-densitydpi=middle-dpi:**         602</code> </p>
  
  <p><code>**target-densitydpi=high-dpi:**           909</code></p>
  
  <p><code>**ignore   target-densitydpi:**           602</code></p>
  
  <p>How to calculate the window.inerWidth? Has something to do with window.devicePixelRatio?
  Thank you!</p>