var elem = document.createElement('div');
elem.style.cssText = 'position:absolute;width:100%;height:100%;opacity:0.3;z-index:100;background:#000';
document.body.appendChild(elem);
<!DOCTYPE html>
<html lang="en">
<head>
<link rel="stylesheet" href="style.css">
<title>DOM</title>
</head>
<body>
<div id="div">
</div>
<script>
const div = document.getElementById("div");
const paragraph = document.createElement("p");
paragraph.textContent = 'Hello. I was created dynamically';
div.appendChild(paragraph);
</script>
</body>
</html>