D9d9jack

<style>
.first {
    position:relative;
    display: flex;
    min-height: 400px;
    align-items: center;
}
.first .centered {
    color:#fff;
    z-index:2;
    width:100%;
    text-align:center;
    z-index:2;
}
.first .video-box {
    position: absolute;
    top: 0;
    bottom: 0;
    width: 100%;
    height: 100%;
    overflow: hidden;
    z-index:1;
}
.first .video {
    min-width: 100%;
    min-height: 100%;
    width: auto;
    height: auto;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);
}
</style>
<div class="first">
    <div class="video-box">
        <video class="video" autoplay="autoplay" loop="loop" preload="auto">
			<source src="header_video.mp4" type="video/mp4">
		</video>
    </div>
    <div class="centered">
        <button onClick="document.getElementsByTagName('video')[0].play();this.remove();">Start</button>
    </div>
</div>