JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>

<meta charset=utf-8 />

</head>
<body>

  <div id="video">
      <object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/_-wDuGDtjCc?fs=1&amp;hl=en_GB"></param><param name="wmode" value="transparent"></param>
<param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/_-wDuGDtjCc?fs=1&amp;hl=en_GB" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>
    </div>
  
    <div id="overlay"></div>
      <div id="modal">Why won't the overlay cover the Flash?</div>
</body>
</html>

CSS

#overlay {
    display: none;
    width: 100%;
    height: 100%;
    position: fixed;
    top: 0;
    left: 0;
    background: #111;
    opacity: 0.95;
    -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(Opacity750)";     
    filter: alpha(opacity=70);
    z-index: 100;
}   
  
  #modal {
    display: none;
    width: 400px;
    height: 50px;
    position: fixed;
    top: 50%;
    left: 50%;
    margin-top: -25px;
    margin-left: -200px;
    z-index: 110;
    background: #fff;
    text-align: center;
    padding: 1.3em;
    -webkit-border-radius: 10px;
    -moz-border-radius: 10px;
    border-radius: 10px;
    font-size: 1.3em;
    line-height: 1.4em;

}


 
  #video {
    position: relative;
    z-index: -1;
  }

JavaScript

$(function() {
  
  setTimeout(function() {
    
    $('#overlay').fadeIn(500, function() { $('#modal').show(); });
    
  }, 1000);
  
});