"This webpage is using significant energy"?
Why? For one, the device I'm browsing on is plugged into a wall, so why do I care if it's using significant energy? Second, maybe design a web browser that can handle web pages rather than the 'solution" of closing it to improve responsiveness. Sure, I can close the website I want to be on, and heck, I can even just turn off my Mac if I really want to save energy.
Why would I want to use a web browser that struggles with the act of browsing the web, and using VERY common websites? I want to stick with Safari, but Google seems to have figured this out.
MacBook Pro 13″