Safari is getting a few updates in iOS 9, but publishers will likely be less than thrilled to hear about one: ad-blocking extensions.

When iOS 9 launches this fall, users will be able to download an extension that blocks ads on most sites. The new browser's Content Blocking Safari Extensions give developers "a fast and efficient way to block cookies, images, resources, pop-ups, and other content," Apple's website said.

According to Safari engineer Ricky Mondello, who tweeted the news on Monday, the extension will work throughout the browser, meaning users won't have to reset it on every page they visit.

Neiman Lab called the new feature "worrisome," arguing that publishers "already make tiny dollars on mobile." And Safari's massive popularityit is by far the most used mobile browser in the U.S.could mean trouble for advertisers.

Content Blocking is also available for Safari on OS X. Developers can use the new Content Blocker section of the Extensions Builder, or the setContentBlocker API.

Cupertino will provide more details during a developer session at its Worldwide Developer Conference (WWDC) at 2 p.m. ET on Friday.

AdBlock Plus, meanwhile, which offers ad-blocking technology, was wary of Apple's announcement. "Either this new API will improve Adblock Plus performance on Safari or it will force us to rely upon an inferior blocking format that would essentially kill adblocking on Safari," the company said.

"According to the announcements there are so called 'block lists,' which are JSON files, that can be registered by the extension," AdBlock said. "We are nervously awaiting how powerful [Apple's] block lists will be." But "if their block list format turns out to be useless, however, that could mean the end of adblocking on Safari."

Apple on Monday provided a sneak peek at the upcoming iOS 9, which is available now for developers to test, ahead of a public beta coming in July.

Among the new iOS features are Intelligence (Apple's answer to Google Now), split-screen iPad multi-tasking, Apple Maps transit directions, an updated Notes app, and more Apple Pay support. Users will also be able to tap into the Flipboard-like News application, which highlights news based on your interests.

For more, see How to Get iOS 9, as well as 11 Intriguing Features in Apple's iOS 9 in the slideshow above.

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