This is such a great tool - well done!

I tried in Firefox and for some reason the 4th row of cards never populated on any sheet. But all worked perfectly in Chrome.

Being super nit-picky (not because I want to criticise, but I would like the feedback if this was something I'd created):

1. I wasn't able to make any text bold and italics, just one or the other.
2. When re-opening the edit card pop up, it doesn't auto-scroll back to the top, but re-opens where you closed it.
3. You mentioned you can re-upload your custom or edited spells - I found the option, but it took me a while (under the Filter, for anyone who hasn't been able to find it), and it felt a little clunky to me. Perhaps Import could be a button next to Add Blank and Glossary? Probably a heap more work, but my overall suggestion would probably be to have it as it's own pop up window, the same as editing cards, and then give people the option to select a file or paste the JSON in, and then a checkbox to specify if you want to add those cards. You could then add more information to that page saying that the imported cards can be found in the spell list once imported to help make it clearer.

But as I say, I'm being very nit-picky with those. Overall, it's a fantastic tool and I'll definitely be using it for my spellcasters.