How to embed JavaScript in a web page?

Question

JavaScript

You need to locally embed JavaScript on the page without being able to edit this page.
In general, some kind of Stylish, but for JS.

Answer 1

Greasemonkey for Firefox:
https://addons.mozilla.org/en/firefox/addon/grease...
and Tampermonkey for Chrome:
https://chrome.google.com/webstore/detail/tampermo...

Answer 2

You open the page, in the address bar you write something like:
With the same success, you can edit anything, even add your own tags