It isn't.

Javascript-based generation is notorious for being subpar as compared to your normal wallets. Having to rely on your browser to correctly communicate with the OS CSPRNG with no assurance isn't very good if you need to ensure that your generation involves enough entropy. JS cryptography modules aren't very well implemented or audited either, there has been cases where users have lost funds due to poor implementation by the developers or just from the limitation of javascript. I wouldn't bother running JS sites, offline or not. The fact that you'll have to trust put your trust in so many different components and have no assurance of whether it would be working perfectly is quite dangerous.

Consider just using a normal desktop wallet for generation instead.