Css background video

Since you are using an HTML5 element the best way to center content is to put it in a relative container and then let CSS handle the rest like this:

<div id="Container">
    <video></video>
    <div></div>
</div>

body, html {
    height: 100%;
}

#Container {
    width: 100%;
    height: 100%;
    position: relative;
    overflow: hidden;
}

#Container video {
    position: absolute;
    left: 50%;
    top: 50%;
    /* The following will size the video to fit the full container. Not necessary, just nice.*/
    min-width: 100%;
    min-height: 100%;
    -webkit-transform: translate(-50%,-50%);
    -moz-transform: translate(-50%,-50%);
    -ms-transform: translate(-50%,-50%);
    transform: translate(-50%,-50%);
    z-index: 0;
}

#Container div {
    position: relative;
    z-index: 1;
}

You can replace <video> by any element you want to center, of course. Because you are using the video element I'm ignoring older browsers as I guess they won't like your page anyway. You also don't have to use the min- values, and it would just center.

Working example with object-fit: cover; More about it here https://developer.mozilla.org/en-US/docs/Web/CSS/object-fit

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  padding: 0;
}

.videobg {
  height: 100vh;
  overflow: hidden;
  position: relative; /* requires for to position video properly */
}

video {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
  z-index: -1;
  object-fit: cover; /* combined with 'absolute', works like background-size, but for DOM elements */
}

<div class="videobg">
  <video autoplay loop muted>
    <source src="http://clips.vorwaerts-gmbh.de/big_buck_bunny.webm" type="video/webm">
    <source src="http://clips.vorwaerts-gmbh.de/big_buck_bunny.mp4" type="video/mp4">
  </video>
</div>