Apple unveiled iOS 9 earlier this week with big improvements to Siri, multitasking on iPad, and an updated News app. While there’s many other additions, including a lowercase keyboard, one change that Apple didn’t highlight is the ability to block web ads in Safari. Apple engineer Ricky Mondello first revealed the change on Monday, and the company is planning to provide more details to developers tomorrow. iOS developers will be able to create extensions for Safari that block cookies, images, resources, pop-ups, and other web content.

Update: Read the Apple iOS 9 review.

Nieman Lab calls the change "worrisome," noting that web publishers are struggling to generate digital advertising revenue through mobile audiences. Apple’s move is significant as the majority of web users in the US use Safari thanks to the popularity of the iPhone. The change also arrives alongside Apple’s News app, which is designed to collate articles from publishers in a similar way to Flipboard. Apple News will be supported by ads, meaning publishers may have to opt in when ad-blocking software inevitably becomes popular in the App Store.