There's no web standard for labeling content, so "subscribe to a whitelist" today means crappy page or network-level content blocks implemented by a third party tool that has no understanding of the actual content on the page or application. That's not at all what's being proposed in this thread.

With a universal labeling standard the UX would be much better, and there'd be a much greater incentive for website owners to participate (they'd be implementing a universal standard, not adding support for one particular parental control tool).