A browser extension is a piece of software installed in a browser to add or change functionality. It can read or modify the pages you visit, depending on the permissions it requests.
Why it matters
Extensions are useful but can affect how sites behave. Ad blockers can hide elements, privacy tools can block scripts, and poorly built add-ons can slow pages or leak data. When a visitor reports a bug you cannot reproduce, an extension is a common cause.
Site owners cannot control extensions on visitors' devices, so build pages defensively and offer a clear way to report problems with a description of the browser used.
Example
A user installs a password manager. It requests permission to read form fields on all sites. If the same page works in a private window with extensions disabled, the add-on is likely interfering.
Common mistakes
- Granting broad permissions such as read and change all your data on all websites without need.
- Debugging a site with a dozen extensions active.
- Installing extensions from outside the official store.
Related
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This page is general information written by our editorial team and updated when documentation changes. Servers and providers differ, so test changes safely first. See our editorial policy and disclaimer.