This is a site I built today on Ideademic in PHP, and of course with Claude. It is a repository of bookmarklets to allow you to tweak your sites. Anybody can post a bookmarklet here.
What is a bookmarklet?
We're going to answer that first. In a browser, a bookmark is a button, usually, that links to a page, commonly over HTTP/HTTPS. But, there are other protocols, including javascript: which runs javascript code in your current tab. A bookmarklet is just a bookmark with a javascript: protocol, which means it runs arbitrary javascript code in the website you are browsing.
Why is this useful?
Bookmarklets are more lightweight than browser extensions and can do less - they can't automatically do stuff, nothing runs in the background, they can't see your search history. They can only do things in the context of the current page. They can still be used maliciously in the way javascript can be, but they have less permissions than a browser extension (even though browser extensions need their permissions given to them.)
What can you do with this?
Literally anything javascript can. You can recolor pages, inject cool tools, and much more. For example, one of my bookmarklets makes the page tilt when you click.
You can also make any page dark mode, extract unique colors, blur elements, and have a spotlight around your cursor with my bookmarklets.
Have fun!