This is a chrome extension that developers can use to make their own extensions for chrome or firefox.
In the extension's pop-up, simply press the ➕ icon to create a new chrome extension. Give your extension a name and description. Define what websites you want the extension to run on. Write vanilla JavaScript to define how the extension will run on these sites. Click the 💾 icon to download the code for the extension, which is ready to be published on the chrome web store or further edited as you see fit.