A browser extension is a small web app with extra privileges: it reads and changes pages, adds toolbar buttons and panels, and reacts to tabs, using the WebExtensions APIs that Chrome 1 , Edge, Firefox 555 and Safari 10 share. Manifest V3 is the only format Chrome still runs: Chrome 138 was the last version able to run Manifest V2, and the Chrome Web Store 1 removed the remaining V2 extensions on August 31, 2026. The subsections below build Link Counter, which counts each page's links and shows them on a badge and in a popup.