title: Media selectors not working in sizes attribute for picture html5 tag
body: <p>I am trying to use the new picture tag to have a responsive version of my pictures. </p>
  
  <pre><code>&lt;picture&gt;
    &lt;source sizes="(min-width: 1320px) calc((100vw - 30px)/12*7 - 30px,
                   (min-width: 1091px) calc((100vw - 30px)/2 - 30px),
                   calc(100vw - 75px)"
            srcset="public/img/TSM/16_128.png 128w,     
                    public/img/TSM/16_256.png 256w,
                    public/img/TSM/16_1024.png 1024w"&gt;
      &lt;img src="public/img/TSM/16_1024.png"&gt;
  &lt;/picture&gt;
  </code></pre>
  
  <p>For some reason, no matter what my browsers width, only the fallback is executed, as the picture width is set to it's formula.</p>
  
  <p>I am using picturefill to enable cross-browser compatibility. If it matters, this is part of a bootstrap-3-column with class "col-md-6 col-lg-7".
  Where is the error in my logic?</p>