The basic functionality is from this tutorial.
Modified it, so that the color of each tile would also move.
Internet Explorer required further modifications, as explained here.
The fix for IE caused problems in Firefox, so had to enclose the IE fix within a browser check for IE.
It now works in current versions of Chrome, Firefox and Internet Explorer.

An interesting article about drag and drop can be found here.
The author points out that drag and drop has been available in Internet Explorer since IE 5.0
and that HTML5 Drag and Drop is based on work done by Microsoft in 1999!