Posted in News
March 8, 2018

How ad-blocking works in Chrome

Google on Feb. 15 turned on the browser ad-blocking feature it’s been talking about for a year. The multi-step process Google has implemented starts with an automated evaluation of a sampling of a site’s pages that tallies violations of the coalition standards. Each site is handed a grade: Passing, Warning or Failing.

A warning means that the site has “a number of ad experiences that violate the Better Ads Standard,” which the site administrator should correct before re-submitting for a follow-up review. A failing mark means “numerous” violations were found, and that the site owner has 30 days to make changes and request a second review. Ad filtering kicks in after that grace period if fixes are not implemented.

Once ad filtering begins, Chrome will scrub advertisements from a site for at least 30 days, because review submissions after the first two cannot be made until 30 days have passed.

Failing-grade sites get added to a list that Google maintains on its servers. Chrome uses that blacklist to look up URLs. When it finds a match – the browser has been steered to a site with a Failed grade – Chrome does another look-up, this time to a set of ad “fingerprints” caged from EasyList, the open-source ad-identification-and-removal rules list that forms the backbone of most browser ad blockers, including Adblock and Adblock Plus.

At that point, Chrome declines to honor the page’s requests for ads, preventing them from appearing because they’re never loaded. (Most ads are served by third-party networks, not directly from the servers running the site on which those ads run.) On any page with at least one omitted ad, Chrome notifies the user with an “Ads blocked” message. That, in turn, can be expanded to display a description which reports, “This site tends to show intrusive ads,” along with an option to turn off the browser’s filtering for that site.

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