Visual dev-warning stolen from the Django project page. https://docs.djangoproject.com/en/dev/

index.html

...

<div id="dev-warning">
	dev-warning
</div>

...

styles.css

#dev-warning {
  font-weight: 600;
  text-align: center;
  position:fixed;
  top: 0;
  width: 100%;
  padding: 5px 7px;
  background-size: 50px 50px;
  background-color: #ffe761;
  color: #000;
  border-bottom: 2px solid #ae9e49;
  font-size: 12px;
  background-image: -webkit-linear-gradient(-45deg, rgba(0, 0, 0, .04) 25%, transparent 25%, transparent 50%, rgba(0, 0, 0, .04) 50%, rgba(0, 0, 0, .04) 75%, transparent 75%, transparent);
  background-image: -moz-linear-gradient(-45deg, rgba(0, 0, 0, .04) 25%, transparent 25%, transparent 50%, rgba(0, 0, 0, .04) 50%, rgba(0, 0, 0, .04) 75%, transparent 75%, transparent);
  background-image: linear-gradient(135deg, rgba(0, 0, 0, .04) 25%, transparent 25%, transparent 50%, rgba(0, 0, 0, .04) 50%, rgba(0, 0, 0, .04) 75%, transparent 75%, transparent);
}