What file extension is commonly associated with external stylesheets?

The file extension commonly associated with external stylesheets is .css. Cascading Style Sheets (CSS) is a stylesheet language used for describing the presentation of a document written in HTML or XML. The primary role of a .css file is to define the styles for a web page, including layout, colors, fonts, and overall formatting. When a web developer creates an external stylesheet, they typically save it with the .css extension to indicate its purpose and also to ensure that web browsers can recognize it as a stylesheet.

The .html extension, used for HTML files, serves as the structure of a web document and does not contain styling information itself. The .js extension is associated with JavaScript files, which are used for adding interactivity and dynamic behavior to web pages, but they are not specifically for styling. Lastly, .xml is a markup language used to store and transport data, which does not have a direct role in styling web documents. Therefore, the .css extension is specifically designed for external stylesheets, making it the correct choice.