Soo... browsers will likely support it until the end of the internet?
Yup, unless HTML6 is backwards-incompatible, which is highly unlikely.
Even then, Browsers will likely support HTML5 until the end of the internet.
Yeah, it's not great UX for web apps, but for a toy project like this it's good enough if you don't want any JS. It still works even if deprecated, and if it stops working, it doesn't take anything away.