mozilla-release/testing/web-platform/tests/html-longdesc/iframe-discoverability-manual.html

<!DOCTYPE html>
<html>
 <head>
  <title>Longdesc test: iframe discoverability</title>
 </head>
 <body>
  <iframe src="external-image-external-description-manual.html" width="480" height="320">
  <p>If there is no document included here with an image, the result of the test is unknowable</p>
  </iframe>
  <p>In order to pass, it should be possible to discover that there is an image with a longdesc included in this document (inside an iframe).
 </body>
</html>