The Power Management WebAPI (https://wiki.mozilla.org/WebAPI/PowerManagementAPI) could be useful for Web Apps on Android. We should make it work.
Per bug 1235869, we're going to disable the Android web runtime, so we won't fix this bug in it. (This is part of a bulk resolution of bugs in the Firefox for Android::Web Apps component, from which I attempted to exclude bugs that are not specific to the runtime, but it's possible that I included one accidentally. If so, I'm sorry, and please reopen the bug!)