Very cool. It would be nice if it was cross-platform, but I understand that that would have been a bit beyond your 5 minute scope. (I use Mac, Windows, and Linux roughly the same amount daily)
Someone on the comments implemented a chrome extension that opens up a gif with the same ratios as breathe (4-4-4). It works great to be honest. Here is the link: https://github.com/b3ting/breathe