Conversation

In my experience, the biggest source of unnecessary energy waste in websites is JavaScript doing some kind of polling while the page is idle. Think about it: you spend ~95% of your time on a web page not doing much: maybe scrolling, maybe typing, maybe literally doing nothing. So if there's something running during "idle" time, consuming CPU, then that will be your biggest battery drain. Especially if you're unfortunate enough to leave the tab open for a long time.