My extension

A simple Chrome extension to let you write custom scripts to modify or interact with the pages you visit.

Installation

Before doing anything else, open a wikipedia page, such as https://en.wikipedia.org/wiki/Polyrhythm for testing purposes. If you have a wide monitor, notice how the body text spawls all the way across and is thus (IMO) hard to read.
Go to about:extensions.
In the top left corner, click the hamburger, and ensure "Extensions" is selected.
In the top right corner, enable "Developer mode".
In the top left corner, click "Load unpacked". And select this directory.
You should see an entry in the main grid on the page now with "My Extension 1.0".
Ensure that the word "Errors" does not appear in red inside that grid box.
In the top left corner, click the hamburger, and go to "Keyboard Shortcuts". Find "My Extension" and select a shortcut for the command "Invoke...". (I use Cmd-j.)
Go back to that wikipedia page, and refresh. If the extension is working, you should see the margins of the content change, and the text become much narrower and more readable.
As another test, go to https://YOURWORKSPACE.slack.com/customize/emoji and enter a substring that causes it to display more than one but not more than, say, three emoji. Then press your selected keyboard shortcut. The extension should auto-download all the displayed emoji with filenames corresponding to the emoji names.

Edit the dispatchTable in background.js according to the comments there. You can put script code as a string in the table directly, or you can give it a filename and put your code for the new site in that file.
Go to about:extensions.
In the grid for "My Extension" click the little refresh icon to reload the extension from disk.
Go to the corresponding site and test your code.
Pro tip: if you put your code in a separate file, then after your first edit to background.js, you don't need to reload the extension every time. The extension fetches the site-specific scripts from disk on every invocation. Only when you edit background.js do you need to reload. (You're gonna forget to reload.)