"Thinking About Power Usage and Websites" by Chris Coyier https://css-tricks.com/thinking-about-power-usage-and-websites/
It's an interesting thought experiment. In general, on the client side, more CPU does map to more energy usage. And Safari's dev tools actually do have an estimate of battery consumption now.
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.
