AdBlock, like all ad blockers, relies on filter rules to know what to block, hide, and (in the case of whitelisted sites) allow to appear on the web pages you visit. AdBlock compares every HTTP (web page) request to the filter lists you're subscribed to and any custom filters you have added. If the URL (web address) of the request matches one of the filters, the request is blocked and the resource is not downloaded.
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.
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.)
- You can also subscribe to additional filter lists that block things other than ads, such as certain annoying aspects of Facebook, social media buttons and in-page pop-ups, and ads on sites in languages other than English.
- You can also subscribe to the Acceptable Ads filter list, which supports websites by allowing some non-intrusive advertising to appear. (Acceptable Ads is enabled by default when you install AdBlock.)
- 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.)
We don't control what goes on the lists. With the exception of Acceptable Ads, all the filter lists are maintained by volunteers in their spare time. You can contact them directly if an ad is not blocked. Acceptable Ads is maintained by Adblock Plus (ABP), our friendly rival, and is under the control of an independent board.
For more information, please see What filter lists should I use?