Acabo de probar tu ejemplo y no me funciona con el video de youtube
   Código PHP:
    <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:og="http://ogp.me/ns#" xmlns:fb="http://www.facebook.com/2008/fbml" lang="es" xml:lang="es">
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<meta name="Language" content="Spanish">
 
 
<script type="text/javascript">
/*<![CDATA[*/
 
var g;
var r;
 
window.onload = function()
{
    g = document.getElementById('gallo');
    r = document.getElementById('relleno');
    g.loop = true;
    r.onmouseover = function() { r.style.opacity= 0; g.play(); };
    r.onmouseout = function()  { g.pause(); r.style.opacity=0.4; };
};
    
 /*]]>*/
</script>
 
<style type="text/css">
 
body { background-color: white;}
 
#marco
{
        position: absolute;
        right: 50%;
}
 
#gallo
{
        position: relative;
        right: -50%;
        z-index: 1;
}
#relleno
{
        position: absolute;
        top: 0px;
        right: -50%;
        z-index: 2;
        width: 100%;
        height: 100%;
        background-color: white;
        opacity: 0.2;
}
 
#web
{
        position: absolute;
        width: 100%;
        height: 2em;
        line-height: 2em;
        text-indent: 2em;
        bottom: 0;
        left: 0%;
        z-index: 3;
        background-color: white;
}
 
</style>
 
<title>Prueba video</title>
 
</head>
 
<body>
    <div id="marco">
        <video id="gallo" class="video-stream html5-main-video" x-webkit-airplay="allow" data-youtube-id="5ukRzvCNTeo" src="http://r19---sn-h5q7dn7k.c.youtube.com/videoplayback?ms=au&ipbits=8&ratebypass=yes&expire=1378169182&source=youtube&sparams=cp%2Cid%2Cip%2Cipbits%2Citag%2Cratebypass%2Csource%2Cupn%2Cexpire&ip=213.97.96.43&fexp=936102%2C924606%2C929117%2C929121%2C929906%2C929907%2C929922%2C929127%2C929129%2C929131%2C929930%2C936403%2C925726%2C925720%2C925722%2C925718%2C929917%2C906945%2C929933%2C920302%2C913428%2C920605%2C919811%2C913563%2C904830%2C919373%2C930803%2C908536%2C904122%2C938701%2C936308%2C909549%2C900816%2C912711%2C904494%2C904497%2C900375%2C934507%2C936312%2C906001&itag=43&key=yt1&cp=U0hWTVFRV19NTENONl9JS0FCOjlFOGVSWTByeVRl&id=e6e911cef08d4dea&mt=1378145357&sver=3&mv=m&upn=PUFx3vy3okc&cpn=MsK8GOyaZ8ShuIb0&signature=BC797E5ECA5D5F5F476A2B3BC8845701BE45CDC0.D17BAF89BC54791A18A60D2D63B01C80E6AF60E6&ptk=youtube_none&pltype=contentugc" style="width: 420px; transform: none;"></video>
        <div id="relleno"></div>
    </div>
</body>
</html> 
   
  ¿Porqué podría ser? 
Gracias compy por el trabajo que estás realizando. 
Saludos.  
Javier