> It's a hard problem as web developers have incentive to use all the horsepower available to them.

Although it's not only the fault of the browser devs, but probably mainly of JS/frontend devs... the result is that leaving a browser window open easily halves battery time, because even innocuous sites eat half a core of CPU. Also, fan's always whirling.

Qubes OS is quite nice here. Just pause the VM with the browser in it -> 0 % CPU, but can still read what's on the display.