How does ad blocking works?

Ad blockers are software programs (plugins or browser extensions) that block or modify advertising content on a website. During the loading of a webpage, the ad blocker examines the site’s scripts and compares them to a list of sites and scripts it was designed to block. It blocks them if it finds any.

A common way of displaying ads is to encode an image using “data:image/png;”. This method doesn’t trigger a normal HTTP request. That means ads called in this way can’t be blocked. Instead, AdBlock injects a stylesheet onto every web page. The stylesheet contains selectors that are set to “display:none !important”, which hides the ads on the page.

Some of these applications replace that advertising content with something else. Others don’t replace it with anything, leaving holes or broken links on the page where the ads would’ve been.

What do ad blockers block? The type of content that is banned varies by app. Some of them are designed to remove all advertisements from a website. Others specifically block things that might endanger a user’s privacy, such as tracking codes that give marketers information about visitors’ activities on a page (like how long a person spends looking at the page or which other pages they look at on the website).

Where do the blocking and hiding filters come from?

Most of the filter rules come from EasyList, which is a huge collection of known ads on English-language websites all over the Web. EasyList is the primary ad-blocking filter list that all ad blockers use. (Starting with Microsoft Edge, we subscribe you to EasyList Lite, not EasyList, when you install AdBlock.)

Additional filter lists can be subscribed to block things other than ads, such as certain annoying aspects of Facebook, social media buttons and in-page pop-ups, and ads on non-English websites.

You can also join the Acceptable Ads filter list, which helps websites by allowing non-intrusive advertising to appear on them. (When you install AdBlock, Acceptable Ads is enabled by default.)

Finally, you can subscribe to still other filter lists if you know their URL (Web address).

You can also add custom filters. These are rules you provide to AdBlock that aren’t in a filter list:

  • You can add them manually, by writing them yourself following established filter rule syntax.
  • You can use our wizard to create custom filters without knowing how to write them yourself.
  • You can also import filters from other ad blockers and from posts in the EasyList forums. (Just visit EasyList’s Report Unblocked Content forum and search for the name of the site where you’re seeing an ad.)


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