<div class="element">width: 300px .... 400px;</div>
.element {
  width: 300px;
  line-height: 150px;
  margin: 50px auto;
  text-align: center;
  background-color: #009966;
  color: white;
  cursor: help;
  transition: width .3s;
}

.element:hover {
  width: 400px;
}


body {
  font: 1.2em monospace;
}

External CSS

This Pen doesn't use any external CSS resources.

External JavaScript

This Pen doesn't use any external JavaScript resources.