title: Automatically play html video when in element becomes visible in viewport
body: <p>Actually I just want to put this as a kind of Wiki, after breaking my fingers on this.</p>
  
  <ol>
  <li>How to automatically start video when in viewport. </li>
  <li>Most importantly: <strong>Wordpress compatible solution</strong> PURE JavaScript. No JQuery. Why? Because newbies like me have a hard time "enqueing" libraries into wordpress... When even just receiving a scroll event is hard...</li>
  </ol>
  
  <p>Anyway, posting answer below:</p>