Overlay play button

<div class="container">
<!-- The video element -->
<video src="..." />
<!-- The overlay area -->
<div class="container__overlay">
<!-- The player button -->
...
</div>
</div>
.container {
/* Used to position the overlay */
position: relative;
}
.container__overlay {
/* Position */
left: 0;
position: absolute;
top: 0;
/* Take full size */
height: 100%;
width: 100%;
/* Center the content */
align-items: center;
display: flex;
justify-content: center;
/* Add a dark background */
background-color: rgba(0, 0, 0, 0.25);
}

See also

Video background