> For example, a well-meaning website performing intensive computation only when the page is visible.
if the user wants the computation to continue, despite being not visible, the site should not be able to override it. It would be anti-user otherwise.
often, i would see video sites _not_ prebuffering when the page is not visible. Or, deliberately switch over to a low resolution version of the video when the visibility changes - twitch does that. Even if i have the video overlay playing outside the browser window (so technically, it is visible, just not according to the API).
Essentially, if the user has the know how to switch this preference (and it would be per site, or even per session, like mic or camera), they would know the consequences.
> require user permissions.
It's not a permission per se, but a toggle, so the user might turn it on or off depending on their circumstances. They don't need to understand it - they can just leave it on default as what the browser currently does if they don't understand, or don't have any problems with the site's behaviour.