They can just not send you the actual content until you've downloaded the advertising content of the page.

And in HTTP/2.0 they'll just switch to pushing the ads out to you.

In either case they can't guarantee it'll get on the screen, but they assuredly can make sure you download the damned ads.

Long term browsers will become more and more closed, DRMed, protected and such so that people have much less control over how stuff is displayed on screen.