greasemonkey

A userscript is a kind of lightweight and less privileged browser extension written in JavaScript used to modify webpages. Userscripts may use the special API, usually referred as GM_API, allowing them to bypass same-origin policy and store data in separate storage not accessible to web pages. The technology was initially introduced in the GreaseMonkey extension for Firefox web browser, and then spread to other browsers, sometimes built-in, like in Presto-based Opera, sometimes with help of extensions, like in Chromium-based browsers. The most famous catalogs of userscripts are GreasyFork and OpenUserJS.