iOS 9 Allows Content Blockers in Safari

What Changed

iOS 9, released in September 2015, let apps from the App Store add content blockers to Safari. A content blocker supplies Safari with a list of rules, and Safari applies them to block ads, tracking scripts, cookies and other resources before they load.¹

Apple designed the system so that the blocker only provides rules and never sees the pages the user visits. Safari does the blocking itself, which also made it fast and efficient on mobile devices.

Blocking was off by default. A user had to download a content blocker app and turn it on in Safari's settings.

Why It Mattered

It was the first time iPhone users could block web trackers at scale, and content blockers went straight to the top of the App Store charts. Peace, from Instapaper's creator Marco Arment, was the top-selling paid app until he withdrew it two days after launch.² Publishers and advertisers worried about lost ad revenue and gaps in analytics. A PageFair and Adobe report published weeks earlier had estimated that ad blocking would cost publishers $21.8 billion worldwide in 2015.³ Blocking on mobile was still rare, and the report expected iOS 9 to change that. PageFair sold tools to counter ad blockers, so its figures were not neutral.

The early measured impact was small. Adobe, after investigating a customer's drop in Safari traffic, found no measurable drop in Safari traffic across its customer base, because only a minority of users installed a blocker.⁴

The bigger significance was the direction it signalled. Content blockers showed that Apple was willing to give users tools against tracking in Safari, and the rule-based blocking engine it built became part of later protections. Two years later Apple stopped relying on users to opt in and turned on Intelligent Tracking Prevention for everyone.