I've built a similar piece of javascript crypto: PasswordProtectMyFile.com. It's a single HTML page that can work with internet turned off. The ciphertext is another HTML page with all the javascript needed to decrypt itself.
It's all in one html file, so it will work disconnected from the internet. Download or save the page, turn off your internet, open it in a browser, and encrypt with peace of mind (although the javascript could still use off-line storage and then next time you're online on the page access it, I guess).
It's open source: https://github.com/louissobel/ppmf