From the title I thought it would be something along the lines of an NaCl implementation in JavaScript. It's actually a software that helps breaking the web for its users. Basically it allows paywalled websites to send users their inaccessible data even of they won't be able to read it. The title should be "DRM for web page that eat up your data plan even if you can't access the content".
Websites could have done this before as well, and many in fact already do. It's generally established pattern in webdev if you need to be able to unlock content in a fast manner, without having to request additional content from a backend. Notably, many P2P networks works the same way regarding private-but-still-distributed content as well.