.parent {
    width: 100vw;
    height: 100vh;
}

.parent video {
    min-width: 100%;
    min-height: 100%;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translateX(-50%) translateY(-50%);
}

<div class="parent">

    <video autoplay muted loop>
        <source src="https://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4"
                type="video/mp4"/>
    </video>

</div>