YouTube Comment Custom CSS Tester is a Chrome extension developed by キィー. This free tool is designed for users who want to customize the appearance of YouTube Live Chat Comment pages using custom CSS. With this extension, users can load their own CSS and view a customized version of the comment page in their web browser. The CSS loaded onto the page can also be debugged using the browser's built-in developer tools.

To get started, users can access the extension's option page to set up their preferences and open a Live Chat comment URL. Once a comment page is opened, the extension's popup page can be used to edit and hot-reload the custom CSS, allowing for real-time changes.