<div id="one">
</div>

.box{
  height: 150px;
  background-color: mediumblue;
  border: 1.5px solid black;
  text-align: center;
  padding: 10px;
}

.box h3 {
  font-size: 150%;
  font-family:Arial;
  color: cornflowerblue;
}

var One = document.getElementById('one');

One.className = 'box';

One.innerHTML = ' <h3>Hello World, I am Learning JavaScript! </h3>';