body {
  margin: 0;
  background: #000; 
}
video { 
    position: fixed;
    top: 50%;
    left: 50%;
    min-width: 100%;
    min-height: 100%;
    width: auto;
    height: auto;
    z-index: -100;
    transform: translateX(-50%) translateY(-50%);
 background: url('http://minio.family/armoiries.gif') no-repeat;
  background-size: cover;
  transition: 1s opacity;
}
.stopfade { 
   opacity: .5;
}

#texte { 
  font-family: Agenda-Light, Agenda Light, Agenda, Arial Narrow, sans-serif;
  font-weight:100; 
  background: rgba(0,0,0,0.3);
  color: white;
  padding: 2rem;
  width: 33%;
  margin:2rem;
  float: right;
  font-size: 1.2rem;
}
h1 {
  font-size: 3rem;
  text-transform: uppercase;
  margin-top: 0;
  letter-spacing: .3rem;
}
#armoiries button { 
  display: block;
  width: 80%;
  padding: .4rem;
  border: none; 
  margin: 1rem auto; 
  font-size: 1.3rem;
  background: rgba(255,255,255,0.23);
  color: #fff;
  border-radius: 3px; 
  cursor: pointer;
  transition: .3s background;
}
#armoiries button:hover { 
   background: rgba(0,0,0,0.5);
}

a {
  display: inline-block;
  color: #fff;
  text-decoration: none;
  background:rgba(0,0,0,0.5);
  padding: .5rem;
  transition: .6s background; 
}
a:hover{
  background:rgba(0,0,0,0.9);
}
@media screen and (max-width: 500px) { 
  div{width:70%;} 
}
@media screen and (max-device-width: 800px) {
  html { background: url(http://minio.family/armoiries.gif) #000 no-repeat center center fixed; }
  #bgvid { display: none; }
}

video { display: block; }

video#bgvid { transition: 1s opacity; }
.stopfade { opacity: .5; }

video#bgvid {
  position: fixed; right: 0; bottom: 0;
  min-width: 100%; min-height: 100%;
  width: auto; height: auto; z-index: -100;
  background: url(http://minio.family/armoiries.gif) no-repeat;
  background-size: cover;
}

var vid = document.getElementById("bgvid");
var pauseButton = document.getElementById("vidpause");
function vidFade() {
  vid.classList.add("stopfade");
}
vid.addEventListener('ended', function() {
  // only functional if "loop" is removed
  vid.pause();
  // to capture IE10
  vidFade();
});
pauseButton.addEventListener("click", function() {
  vid.classList.toggle("stopfade");
  if (vid.paused) {
    vid.play();
    pauseButton.innerHTML = "Pause";
  } else {
    vid.pause();
    pauseButton.innerHTML = "Paused";
  }
})

<button id="vidpause">Pause</button>