This is great, congratulations! I have been working on this as well. We have to make account self-custody both more user-friendly and more secure. I think the offline usage makes it more secure and the BIP-39 standard makes it more user-friendly. But one of the challenges is that most people wouldn't download a tool from a code repository. I like how you have made it into a single HTML file so it can be downloaded - would it make sense to make a Download button so less technical people can more easily download it? And also, sucks that this method doesn't work on iOS.

In any case, kudos for tackling this! Very much needed if we want to expand the scope of people who use blockchain and own their accounts.