Thanks for your work on this! I recently used 98.css in my fairly simple web app[0] for a recreation of something from my childhood, and it worked beautifully for my purposes.
I'd suggest using checkboxes instead of radio buttons for the operator selection. It would be useful, I think, to have a mix of just addition and subtraction, or just multiplication and division, instead of having one option or all four.
I hope you're feeling better. Interesting how retro computing, like old games and operating systems, can be comforting and even healing. I've had similar experience with emulators - playing Gameboy and Nintendo games from childhood, or running Macintosh System 9.
Reminds me of Pico-8, SerenityOS, TempleOS. There's a common thread of a retro-style computing environment that can be entirely understood and controlled by the user.
I imagine there's deeper psychological dynamics to this, like a safe "sandbox" as a therapeutic tool.
It also says something about design with empathy and focus on user experience. There are so many hostile dark patterns these days in technology and society, that it's a relief to find some space (physical or mental) that's designed for your comfort.
[0]: https://www.mathsheets.net/