How Do Ad Blockers Work? What They Block and How Browsers Remove Ads

Ads on the web are not always part of a page in the same way as its headline, text, or images. Many are loaded through separate scripts, frames, network requests, and advertising platforms. That gives browsers and ad-blocking tools several opportunities to stop them before they reach the screen.

Most modern ad blockers combine network filtering with page-level filtering. They can prevent known advertising or tracking resources from loading and then remove unwanted elements that remain in the page layout.

The result may look simple when an ad disappears, although the browser can be checking many URLs and page elements against filtering rules while the site loads.

What Happens When an Ad Blocker Filters a Page?

An ad blocker is a browser feature, extension, or content-filtering tool that identifies unwanted web resources and prevents them from loading or appearing.

A simplified version of the process looks like this.

‍In Chromium-based browsers, extensions can use the Declarative Net Request API to create rules for blocking, redirecting, or modifying matching network requests. The browser evaluates those rules during loading, which allows extensions to filter requests without inspecting every request through extension code.

Safari provides its own content-blocking system. Its rules can prevent matching resources from loading and can also hide selected elements on a webpage.

How Manifest V3 Changed Ad Blocking in Chrome

Chrome's move from Manifest V2 to Manifest V3 changed how browser extensions can filter network traffic.

Under Manifest V2, extensions could use the blocking version of the webRequest API and make filtering decisions while requests were being processed. This gave advanced blockers considerable flexibility over how and when requests were handled.

Manifest V3 moved most extension-based blocking toward declarativeNetRequest. Extensions provide filtering rules in advance, and Chrome applies those rules inside the browser. Google says this approach reduces the amount of browsing data extensions need to access and avoids some of the performance costs associated with running extension code for every request.

The change also affects what advanced content blockers can do. Declarative filtering has quotas and predefined capabilities, while the previous model allowed more filtering logic to run dynamically. Chrome has expanded these limits over time, but the two architectures still work differently.

uBlock Origin is a useful example. The full version of uBlock Origin remained a Manifest V2 extension, while uBlock Origin Lite was created as a separate Manifest V3-compatible version. Its own documentation describes Lite as a reduced version designed around the limitations and permission model of Manifest V3. Some filtering modes in uBlock Origin Lite also require broader site permissions to enable cosmetic and scriptlet-based filtering.

Chrome disabled Manifest V2 extensions for regular users in 2025, and Chrome 139 removed the remaining support path. As a result, Chrome users who want uBlock's filtering approach now use the Manifest V3-compatible Lite version rather than the original extension.

How Does an Ad Blocker Know What Is an Ad?

Ad blockers rely heavily on filter lists that contain rules for domains, URL patterns, scripts, page elements, and other signals commonly associated with ads or trackers.

One rule may match requests sent to a known advertising domain, while another can target a URL pattern used for tracking pixels. Cosmetic rules can identify specific page elements and hide them after the page is rendered.

During loading, the blocker compares relevant requests and elements with its active rules. A match can trigger a blocking or hiding action.

Filter lists also have to evolve over time. Advertising networks change their infrastructure, websites redesign their layouts, and tracking techniques develop. Regular updates help blockers keep up with those changes.

What Is an Ad Blocker Filter List?

A filter list is a collection of instructions that tells an ad blocker what it should look for.

Network rules can target advertising servers, tracking endpoints, and particular URL patterns. Cosmetic rules work at the page level and can hide elements such as advertising containers or overlays.

Different blockers may produce different results on the same website because they can use different lists, apply different defaults, update their rules at different times, or include their own filtering logic.

Chrome's extension platform uses rule-based filtering for extensions that need to block or modify requests.

Blocking an Ad Before It Loads

Network filtering can stop an unwanted resource before the browser downloads it.

Imagine that a webpage requests an advertising script from an external server. A filtering system can compare the request with its active rules before the download proceeds. When the request matches a blocking rule, the browser can stop it.

This can reduce the amount of unnecessary content the browser has to download and process. The same mechanism may apply to scripts, images, frames, tracking pixels, media files, and other network resources depending on the browser and the blocker.

Chrome's Declarative Net Request API and Safari's content-blocking system both support rule-based filtering of web requests.

Why Do Ad Blockers Also Hide Page Elements?

Stopping an advertising request does not always leave the webpage looking clean. A site may reserve a large section of its layout for an ad, so the empty container can remain even after the advertisement itself has been blocked.

Cosmetic filtering handles this part of the process. It identifies selected elements in the page layout and hides them so that empty ad spaces, overlays, and similar elements do not remain visible.

Safari's content-blocking rules support this type of element hiding through CSS selectors.

A single blocked ad can therefore involve both network filtering and page cleanup. The request is stopped during loading, and any leftover container can be removed from the visible layout afterward.

What Can Ad Blockers Block?

The exact result depends on the blocker and its active rules. Common targets include display advertising, pop-ups, advertising scripts, tracking pixels, embedded ad frames, and requests sent to known advertising or analytics services.

Advertising and tracking technologies frequently overlap. When an advertising request also carries tracking data, blocking that request can reduce both the visible advertising and the associated tracking.

What Can Ad Blockers Miss?

No filtering system catches every advertisement on every website.

Some ads are delivered through the same infrastructure as ordinary site content, which makes aggressive filtering more likely to interfere with legitimate functionality. Native advertising and sponsored content can also be embedded directly into an article, making them harder to identify through conventional network rules.

Video platforms may use their own delivery systems, websites can insert promotions dynamically, and publishers can change their implementation when existing filters begin blocking it. Some sites also detect blocked resources and respond with anti-adblock messages.

A more useful measure of an ad blocker is how consistently it removes unwanted content while keeping the websites you use functional.

What Is a Content Blocker?

A content blocker is a broader filtering tool that can control several kinds of web resources. Ad blocking is one of its most common uses.

Depending on the browser and implementation, content-blocking rules may stop network requests, hide page elements, remove unwanted resources, or change how selected requests are handled.

Apple uses the term content blocker for Safari's declarative filtering system. Safari evaluates the supplied rules inside the browser, which means the extension itself does not need a history of every website the user visits in order to make individual filtering decisions.

In everyday use, the terms overlap because many content blockers are installed primarily for ads and trackers.

Built-In Ad Blocker vs Browser Extension

Ad blocking can be included directly in a browser or added through an extension.

Extensions work through the APIs and permissions that the browser exposes. Chromium extensions commonly use declarative filtering rules to block or modify matching requests.

A built-in blocker is integrated into the browser itself. Users can start with filtering already available and manage it through the browser's own settings.

Dedicated extensions often provide deeper customization through additional filter lists, custom rules, and detailed per-site controls. Browser-native blocking can offer a simpler setup with tighter integration into the browser's privacy features.

Do Ad Blockers Also Block Trackers?

Many ad blockers also stop tracking resources because advertising systems frequently include tracking components. Filter lists can contain dedicated rules for analytics domains, tracking pixels, and other services that collect browsing data.

Tracking can also occur on pages with no visible advertising. Analytics scripts, invisible pixels, and third-party services may operate entirely in the background. This is why privacy-focused browsers often treat ad blocking and tracker protection as related features with separate roles.

Sigma includes a built-in ad blocker alongside tracker protection as part of its privacy features.

Can Ad Blocking Make Websites Faster?

Ad blocking can improve loading performance on pages that rely heavily on advertising and tracking resources.

A modern webpage may request images, scripts, video, analytics systems, advertising auctions, tracking pixels, and other third-party resources alongside the content the visitor actually wants. Preventing some of those requests from loading reduces the amount of data the browser needs to download and process.

The effect varies between websites. A lightweight page with a small amount of advertising may show little difference, while a page filled with third-party scripts and ad resources can benefit much more.

Blocking unnecessary resources can therefore reduce page workload and data transfer, although the improvement depends on what the site would otherwise load.

How Sigma Handles Ads and Trackers

Sigma includes ad blocking directly in the browser, so users can access basic filtering without installing another extension.

Sigma's privacy features combine ad blocking with tracker protection. This gives users a cleaner browsing experience while reducing some of the advertising and tracking requests that would otherwise load in the background.

For users who want everyday filtering without managing additional extensions, the main benefit is convenience. Ad and tracker protection are available as part of the browser's normal privacy setup.

Do You Still Need an Ad Blocker Extension?

The answer depends on the level of control you want.

A browser with effective built-in blocking may already cover everyday needs such as intrusive advertising and common trackers. Users who want custom filter lists, advanced rules, detailed per-site controls, or a particular filtering setup may still prefer a dedicated extension.

Using several overlapping blockers can also make troubleshooting more difficult. If a website stops working, identifying the rule or tool responsible becomes harder when several filtering systems are active at the same time.

For most users, one blocker that provides the right balance of coverage and control is usually enough.

Final Takeaway

Ad blockers identify unwanted web resources and prevent them from loading or appearing on the page.

Filtering usually begins while the browser is requesting scripts, images, frames, and other resources. Rules can recognize known advertising or tracking patterns and stop matching requests before they finish loading. Cosmetic filtering can then clean up empty containers and other unwanted elements that remain in the page layout.

Extensions and built-in browser blockers can differ in how much control they provide and how deeply they integrate with the browser. Their practical goal remains similar. They reduce unwanted advertising and tracking while trying to keep websites working normally.

Ads on the web are not always part of a page in the same way as its headline, text, or images. Many are loaded through separate scripts, frames, network requests, and advertising platforms. That gives browsers and ad-blocking tools several opportunities to stop them before they reach the screen.

Most modern ad blockers combine network filtering with page-level filtering. They can prevent known advertising or tracking resources from loading and then remove unwanted elements that remain in the page layout.

The result may look simple when an ad disappears, although the browser can be checking many URLs and page elements against filtering rules while the site loads.