I'm rather sad that for example Battery API is deprecated... possibly might need a browser-wide toggle, but still...
It can both be cool, and useful....mostly cool, though...
you could have a complete web desktop (there several web desktops, and even more "fake" ones, like windows93) that shows the status, or a webpage that switches to dark mode with minimal javascript and theming, when battery is low, or a low distraction writing environment for example, that still notifies you of low battery, etc...
more and more people are using battery-powered devices ;)