title: SVG mm and px co-ordinates lengths differ for defined viewBox
body: <p>In the following SVG image I expected the two rectangles to be at the same location and have the same size but they do not. </p>
  
  <p>My understanding of the viewBox attribute is that in this instance  10px should be equal to 1mm. Is this correct or am I missing something?    </p>
  
  <pre><code>&lt;?xml version="1.0" standalone="yes"?&gt;
  &lt;svg 
      width="210mm" 
      height="297mm" 
      viewBox="0,0,2100,2970"
      preserveAspectRatio="none"        
      xmlns="http://www.w3.org/2000/svg"&gt;
          &lt;rect x="10mm" y="10mm" width="50mm" height="50mm"
              fill="rgba(155,155,155,0.5)" id="x1" &gt;&lt;/rect&gt; 
          &lt;rect x="100" y="100" width="500" height="500"
              fill="rgba(155,155,155,0.5)" id="x2" &gt;&lt;/rect&gt; 
  
      &lt;/svg&gt;
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/GR8wu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GR8wu.png" alt="resulting image"></a></p>
  
  <p>Thanks in advance!</p>