Showtime websites secretly mined user CPU for cryptocurrency

What if there was a spec that web browsers could submit small amounts of mining work as a micropayment system for web content? (amount determined by content provider per-page or per-session or some other better granularity, only sent if user approves)

Browsers could pre-mine/cache some amount of work so that page load times would be relatively unaffected.

Browsers could ask about payment similar to pages requesting location (yes/no/always/never) so future payments are automatically applied or denied.

Thoughts?