I'd be very interested to hear from a Lawyer what are the implication of doing something like this. Controlling the last mile of fruition (in this case the bro

I'd be very interested to hear from a Lawyer what are the implication of doing something like this. Controlling the last mile of fruition (in this case the browser) can open a lot of opportunities (ie.: replacing podcast ads with contextual one, etc). But I wonder if this is legal or not.