Here's a longer write up on the hardware involved and thinking behind it. It's linked from the article but I thought this explained it better.<p><a href="http://blog.nytlabs.com/2013/11/05/diff-a-live-comparator-for-the-web/" rel="nofollow">http://blog.nytlabs.com/2013/11/05/diff-a-live-comparator-fo...</a>
Very similar to Berg's Little Printer<p><a href="http://bergcloud.com/littleprinter/" rel="nofollow">http://bergcloud.com/littleprinter/</a>