simple diagonal background with css

diagonal.css

span {
  background: url('data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAKElEQVQIW2NkQAOvX7/+z4gsBhIQFRVlhAvCBECKwILIAmBBdAGQIAACIhPGO7psSwAAAABJRU5ErkJggg==');
  border: 1px solid #eee;
  box-sizing: border-box;
  display: inline-block;
  height: 150px;
  margin: 5px;
  width: 150px;
}