title: <link rel=preload> must have a valid `as` value
body: <p>I'm trying to preload a video using the <code>link</code> tag's preload rel value per <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content" rel="noreferrer">mdn's documentation on preload</a>. </p>
  
  <p>in my index.html file I'm adding the following to the head:</p>
  
  <pre><code>&lt;link rel="preload" as="video" type="video/mp4" href="video/2_1.mp4" /&gt;
  </code></pre>
  
  <p>In chrome this works fine and preloads the file without issue. </p>
  
  <p>When I open the page in safari 11.3 either on my desktop or on an iphone I get a console error message:</p>
  
  <blockquote>
    <p> must have a valid <code>as</code> value</p>
  </blockquote>
  
  <p>According to the <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content#What_types_of_content_can_be_preloaded" rel="noreferrer">"what types of content can be preloaded"</a> section of the documentation that contains the list of valid values I'm definitely using the correct <code>video</code> type. </p>
  
  <p>I checked both the <a href="https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link#Browser_compatibility" rel="noreferrer">mdn documentation</a> for mobile safari preload option on the link tag and it shows a "compatibility unknown" question mark. I also checked<a href="https://caniuse.com/#search=preload" rel="noreferrer">caniuse</a> and it seems to indicate that as long as my mobile safari version is at 11.3 I should be able to use it. </p>
  
  <p>The phone and my desktop are both at safari 11.3, so I'm not sure why I'm getting this error. </p>
  
  <p>Any ideas/insight??</p>