This is really cool, and I'd like to encourage work like this. I currently don't think I can recommend this to others because: It isn't immediately obvious who is running the site. whois info lists a Canadian privacy company. The site doesn't have an about page. Github has contributor info, which may help. Tax returns need to be private and accurate. If it's hard to verify that the authors are even in the US, it's ha…
The site runs client side only, with no external API calls, so assuming you trust the code in the repo is the code being served to you then no data can leave your computer. If you're curious you could probably verify by clicking around on the site and checking the network calls made to ensure there's no external API calls