How is an external style sheet referenced in an HTML file?

An external style sheet is referenced in an HTML file using a link tag, which is placed within the head section of the HTML document. This link tag includes the rel attribute to specify the relationship between the HTML file and the linked CSS file, as well as the href attribute that provides the path to the external style sheet.

This method allows for separation of content and presentation, making it easier to maintain and update styles across multiple pages of a website by changing a single CSS file, rather than altering each HTML document individually. Using an external style sheet promotes consistency in design and improves loading speed by allowing the browser to cache the styles separately from the HTML.