title: How to use the <svg> viewBox attribute?
body: <p>I am learning svg from its official documents, there is such line. I don't get it, if it already has a <code>width</code> and <code>height</code> attribute, what is the point to specify it again in <code>viewBox="0 0 1500 1000"</code> ? 
  It is says, "One px unit is defined to be equal to one user unit. Thus, a length of "5px" is the same as a length of "5"" in the official docs, thus this viewBox is a 1500px wide and 1000 height view, which exceeds 300px and 200px. So why does it define the width and height value in the first place?  </p>
  
  <pre><code> &lt;svg width="300px" height="200px" version="1.1"
           viewBox="0 0 1500 1000" preserveAspectRatio="none"
           xmlns="http://www.w3.org/2000/svg"&gt;
  </code></pre>