This site showed me a popup telling me that I had less than 10% battery life left and that this site could accelerate the drain, which I thought was hilarious.
The battery API was rolled out by all the major browsers between 2012 and 2014 as a way to let web pages avoid doing compute-intensive stuff on low battery, but it was pretty much immediately used for fingerprinting as the battery level number was so precise that if you hit two web pages at the same time, the battery level was as good as a cookie. Firefox and Safari/Webkit disabled it in 2016 but Chrome just capped the precision at two digits.
