This can run offline if you download all the files, as all the computation is done client-side.

However due to the use of require.js (http://requirejs.org) the script files aren't directly linked from the main page. You need to grab the following files, put them in their respective directories locally, and then edit the main page so all the src and href attributes are relative rather than absolute to root (e.g. src="js/lib/require.min.js"):

http://www.sixtylarge.com/index.html

http://www.sixtylarge.com/img/favicon.png

http://www.sixtylarge.com/css/styles.css

http://www.sixtylarge.com/img/sl-logo.svg

http://www.sixtylarge.com/img/hamburger.svg

http://www.sixtylarge.com/js/main.js

http://www.sixtylarge.com/js/script.js

http://www.sixtylarge.com/js/coinList.js

http://www.sixtylarge.com/js/coins.js

http://www.sixtylarge.com/js/secretKeyGenerator.js

http://www.sixtylarge.com/js/seedRNG.js

http://www.sixtylarge.com/js/util.js

http://www.sixtylarge.com/js/walletGenerator.js

http://www.sixtylarge.com/js/lib/analytics.js

http://www.sixtylarge.com/js/lib/bitcoinjs-min.js

http://www.sixtylarge.com/js/lib/jquery-2.1.0.min.js

http://www.sixtylarge.com/js/lib/require.min.js

And there you have it, a nifty offline wallet generator without the risks of running code that could change unexpectedly (especially as there is no https version so greater risk of MITM).