A Pen created at CodePen.io. You can find this one at http://codepen.io/aptary/pen/GLqBt.

 The classic optical illusion created using CSS transitions and animations, no javascript. Stare at the X in the middle of the circle for about 15 seconds and let me know what happens!
Built with SCSS so you can easily change variables like speed and color. Why not fork this and see which other colours work well!
You can read more about this illusion on Wikipedia: https://en.wikipedia.org/wiki/Lilac_chaser