Outliner is a free Chrome add-on developed by Tanguy Antoine. It falls under the category of browsers and specifically the subcategory of add-ons and tools. This handy tool allows users to instantly show the HTML structure of any webpage they are viewing.

With Outliner, users can easily visualize the structure of a webpage's HTML elements. By simply pressing the hotkey "ctrl + m", the add-on generates a clear and organized outline of the HTML structure, making it easier for developers and designers to understand and navigate through the code.

This add-on is particularly useful for web developers and designers who need to quickly analyze the structure of a webpage. Outliner eliminates the need to manually inspect the code or use other complex tools, providing a simple and efficient way to view the HTML structure.

Overall, Outliner is a valuable Chrome add-on for anyone working with HTML code. Its ability to instantly show the HTML structure of a webpage makes it a time-saving tool for developers and designers.