CSS

.overlay {
width: 0%;
height: 0%;
top: 22%; // relativ Mittig
left: 25%; // relativ Mittig
position: absolute;
overflow: hidden;
background-color: white;
transition:
};

Java-Script:

function showOverlay() {
let overlay = document.getElementById("overlay");
overlay.style.width = "50%";
overlay.style.height = "50%";
}