Writing a Browser Extension

Web pages are mostly just a collection of HTML, CSS, and JavaScript, so if we had some way of adding some of these into a web page, perhaps from our browser we could add new behaviour to a web page, right?

Yes; users have long used tools like Greasemonkey (or similar userscript managers) to inject scripts into pages. Better still, modern browsers expose JavaScript APIs that let us interact directly with the browser itself. Enter: browser extensions.