Browsers don't just let you bookmark web pages. You can also bookmark JavaScript, allowing you to do so much more than merely save pages.

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Bookmarklets are JavaScript scripts saved as browser bookmarks, prefixed with `javascript:`, that let you run custom code on any webpage without installing extensions. This guide covers how to create them using IIFEs, URL-encode them for cross-browser reliability, and install them in Chrome, Firefox, and Safari. It also covers CSS bookmarklets using `document.createElement('style')` or the CSSStyleSheet interface, key limitations like Content Security Policy restrictions and browser URL length caps (65536 bytes in Firefox/Safari, ~10M chars in Chrome), and security considerations when running third-party bookmarklets. Alternatives like TamperMonkey userscripts, browser snippets, and extensions are mentioned for more complex needs.

A Complete Guide to Bookmarklets