title: iframe src allows all origins but still receiving Cross-Origin Errors
body: <p>I manage the frontend of siteA and have an iframe on the page with a src pointing to a resource from siteB. This is meant to be an embeddable resource (its a video embed) used by other vendors &amp; clients so the response headers of siteB are set to Access-Control-Allow-Origin *. And all URLs involved are https.</p>
  
  <p>I have a screenshot here of the headers here from the src url in dev tools: </p>
  
  <p><a href="https://i.stack.imgur.com/8hUJl.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8hUJl.png" alt="screenshot of response headers"></a></p>
  
  <p>However, <strong>I’m still receiving cross-origin errors when trying to query the iframe document to check if a specific element is present</strong>.</p>
  
  <p>I’m not trying to modify any content but want to make simple UI decisions depending on the content. </p>
  
  <p>To no avail I’ve tried:</p>
  
  <ul>
  <li>standard jQuery methods like $(‘iframe’).contents().find()</li>
  <li>$.get requests to retrieve the html response and see if it includes a specific string</li>
  </ul>
  
  <p>Am I missing something? Getting over this hurdle would be a major win for our project. </p>