const element = document.getElementById("container");
let text = "H: " + element.offsetHeight + " px | ";
text += "W: " + element.offsetWidth + " px";
document.getElementById("hw_size").innerHTML = text;
The inside container background color is a css root color variable :
root {--blue:lightblue;} #container {background-color:var(--blue);}
position:relative; top:50%; transform:translate(0,-50%);