Apple's latest operating systems for the Mac and iPhone will soon be rolling out, and with that comes new restrictions on ad-tracking in the Safari browser.

Adding a 24-hour limit on ad targeting cookies is good for privacy under Apple's new "Intelligent Tracking Prevention" feature. But if you're an advertiser, the macOS High Sierra and iOS 11 Safari browsers spell gloom and doom for the Internet as we know it. The reason is because Safari is making it harder for advertisers to follow users as they surf the Internet—and that will dramatically reduce the normal bombardment of ads reflecting the sites Internet surfers have visited earlier.

Six major advertising groups have just published an open letter blasting the new tracking restrictions Apple unveiled in June. They say they are "deeply concerned" about them:

The infrastructure of the modern Internet depends on consistent and generally applicable standards for cookies, so digital companies can innovate to build content, services, and advertising that are personalized for users and remember their visits. Apple's Safari move breaks those standards and replaces them with an amorphous set of shifting rules that will hurt the user experience and sabotage the economic model for the Internet.

Apple's unilateral and heavy-handed approach is bad for consumer choice and bad for the ad-supported online content and services consumers love. Blocking cookies in this manner will drive a wedge between brands and their customers, and it will make advertising more generic and less timely and useful.

The letter is signed by the American Association of Advertising Agencies, the American Advertising Federation, the Association of National Advertisers, the Data & Marketing Association, the Interactive Advertising Bureau, and the Network Advertising Initiative.

Apple WebKit engineer John Wilander said that Intelligent Tracking Prevention "reduces cross-site tracking by further limiting cookies and other website data."

Here is how it works, according to Wilander:

Intelligent Tracking Prevention collects statistics on resource loads as well as user interactions such as taps, clicks, and text entries. The statistics are put into buckets per top privately-controlled domain or TLD+1.

A machine learning model is used to classify which top privately-controlled domains have the ability to track the user cross-site, based on the collected statistics. Out of the various statistics collected, three vectors turned out to have strong signal for classification based on current tracking practices: subresource under number of unique domains, sub frame under number of unique domains, and number of unique domains redirected to. All data collection and classification happens on-device.

Let's say Intelligent Tracking Prevention classifies example.com as having the ability to track the user cross-site. What happens from that point?

If the user has not interacted with example.com in the last 30 days, example.comwebsite data and cookies are immediately purged and continue to be purged if new data is added.

Wilander notes that "if the user interacts with example.com as the top domain, often referred to as a first-party domain, Intelligent Tracking Prevention considers it a signal that the user is interested in the website and temporarily adjusts its behavior," as depicted in this timeline:

As an example, he says, if a Web surfer "interacted with example.com the last 24 hours, its cookies will be available when example.com is a third-party. This allows for 'Sign in with my X account on Y' login scenarios." The good news, Wilander added, is that "users only have long-term persistent cookies and website data from the sites they actually interact with and tracking data is removed proactively as they browse the Web."

Apple's iOS 11 launches September 19 and macOS High Sierra on September 25.