When referencing a CSS file, which attribute specifies the relationship of the linked document?

The relationship of a linked document in the context of an HTML document and a CSS file is specified using the 'rel' attribute. This attribute is crucial because it informs the browser about the relationship between the current document and the linked resource. In the case of a CSS file, the 'rel' attribute typically takes the value "stylesheet," indicating that the linked document is a stylesheet that will be applied to the current page.

The 'href' attribute is essential as it contains the URL of the linked CSS file but does not specify the relationship itself. The 'type' attribute can denote the media type of the linked document, which is also important but not directly related to defining the relationship. The 'title' attribute is used to provide additional information about the linked document but does not play a role in establishing how the documents are related.