The thing is, web developers should test their pages for CPU usage the same as app developers do. And anytime a page is idle, CPU usage should be at 0%. Same as with any other app.
Last quarter notebooks outsold desktops for the first time. Netbooks, and iPhones have been exploding in popularity as well. That means a significant number of your website’s visitors will be running off a battery, and since battery life decreases proportionally with CPU load, you really do owe it to your users to do this. The internet shouldn’t be something you have to be plugged into a power outlet to use.