What problem is this trying to solve? Having a single webpage with full control of the device, but limiting how many resources the ads on it can take? I guess that could be useful on the mobile web. However, the proposed solution is still way too low-level for web developers.

If the problem you're trying to solve is that web pages take up way too many resources (they do), control must be exerted by the browser, on the user's behalf. The only input the web developer should be able to provide is what to prioritize when resources are curtailed. Ideally, all web pages should be stopped completely when they are not the active tab, with background processing allowed for specific purposes, such as receiving notifications for messaging, or playing music. But then the specification should allow the developer to indicate what they need the resources for, instead of just saying "I want some CPU just because". And the primary concern should be how to keep developers from just saying "I want ALL the resources ALL the time" by default until the whole scheme is useless.