It's opensource insofar that the javascript is not minified or obfuscated. You can see it at https://github.com/rebane2001/xikipedia too.
Perfect! Thanks for clarification. I thought there was server-side preparation of the content, but it seems from the other posts that it's all local, and I commend you for that.
I want to try reimplementing it for Wikipedia in another language, would you mind sharing how you went from the 400MB Wikipedia export to the .1x (40MB) file that is downloaded here?