<!DOCTYPE>

<html>
  <head>
    <title>Mr. Skeltal</title>
  </head>
  <body>
    <div style="position: fixed; z-index: -99; width: 100%; height: 100%">
      <iframe frameborder="0" height="100%" width="100%" src="https://youtube.com/embed/q6-ZGAGcJrk?autoplay=1&controls=0&showinfo=0&autohide=1"></iframe>
    </div>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Mr. Skeltal</title>
  </head>
  <body style="margin: 0px;">
    <div style="z-index: -99; width: 100%; height: 100%">
      <iframe frameborder="0" height="100%" width="100%" src="https://youtube.com/embed/q6-ZGAGcJrk?autoplay=1&controls=0&showinfo=0&autohide=1"></iframe>
    </div>
</body>
</html>

Fix HTML syntax and remove top/left margins
Fix HTML syntax and remove top/left margins