The hits just keep coming for advertisers looking to reach consumers through digital ads.

The latest setback for online advertising: the version of Safari that will ship with iOS 9 will make it possible for developers to build ad blocking extensions.

As reported by NiemanLab, developer documentation for iOS 9 details how developers will have new tools for building ad blockers. The documentation explains:

The new Safari release brings Content Blocking Safari Extensions to iOS. Content Blocking gives your extensions a fast and efficient way to block cookies, images, resources, pop-ups, and other content.

Your app extension is responsible for supplying a JSON file to Safari. The JSON consists of an array of rules (triggers and actions) for blocking specified content. Safari converts the JSON to bytecode, which it applies efficiently to all resource loads without leaking information about the user’s browsing back to the app extension.

Noting that AdBlock is the most popular Safari extension on desktop, NiemanLabs’ Joshua Benton suggests that it won’t be long before iPhone and iPad users flock to download ad blockers for their mobile and tablet Apple devices.

And he’s probably right. Ad blockers are no longer tools reserved for the tech-savviest internet users; they’re mainstream.

A setback for mobile advertising?

The rise of ad blockers has forced advertisers and the publishers that serve them to adapt. Native ads, for instance, help advertisers integrate their messages into publisher experiences. They’re not perfect, and they bring with them their own set of concerns, but they’re largely impervious to ad blockers.

But there is never likely to be enough native ad inventory to replace the display and video inventory that is effectively being eliminated by ad blockers, and the possibility that Apple’s new iOS 9 functionality will lead to even greater use of ad blocking tools is a worrisome one for advertisers.

After all, advertisers are investing heavily in mobile and Apple’s devices, including the iPhone and iPad, are some of the most widely used mobile devices on the market. If use of ad blockers becomes widespread on these devices, it could eventually reshape the mobile ad market.

So what are advertisers to do? Ultimately, they need to accept the inconvenient truths about their ads and rethink how they engage consumers online.