Thanks, Yann!

I don't know why, but the first time I tried it, it didn't work. It worked when I tried it again. *shrug*

This is a better solution than just disabling the button (and a better solution than most people arguing with me on this), however, there are still at least three issues with it.

One is the color contrast of the button (which can be easily styled), another is that while this works with the Tab key, it doesn't work with the "f" key when using a screen reader, and finally a tooltip is not the best way to provide the information the user needs.

That's because it's likely the tooltip will need to be a decent amount of text to explain what the user needs to do to use the button. There are other issues with tooltips that Adam Silver enumerates in the following article: https://adamsilver.io/blog/the-problem-with-tooltips-and-what-to-do-instead/

I'd also suggest that you give some additional visual cue on the button (either symbol or text) to let the user know it's disabled.

There's no compliance rule against doing it your way. I'm just shooting for the most accessible solution(s).

All that said, I'd still rather see your solution implemented than simply disabling a button.

Thanks again!