Ad-blockers may seem like an answer to an internet user’s prayers. No annoying pop-ups, no promos before videos and no concerns about accidentally clicking on a virus. But for ad-driven websites, they’re a revenue leech that needs to be pried off. This is becoming one of the internet's biggest debates. More countries are cracking down on ad-blockers—the EU’s European Commission even proposed a rule this week that would allow media companies to ban users who use ad-blockers. The debate is no longer on the fringes with tech’s biggest names, including Facebook, weighing in. “Ads support our mission of giving people...