Which HTML element is commonly used to include CSS styles in a web document?

The element commonly used to include CSS styles in a web document is the "link" element. This element is specifically designed to link external stylesheets to an HTML document, allowing for better organization and reuse of styles across multiple pages. By using the "link" element within the head section of the HTML document, you can specify the location of a CSS file stored separately from the HTML itself, which helps keep the code modular and clean.

The "link" element uses attributes such as "rel" to define the relationship to the linked file, typically set to "stylesheet" for CSS, and "href" to provide the URL of the CSS file. This method of including CSS is preferred for larger projects or when the same styles are needed across multiple pages, as it reduces redundancy and makes maintenance easier.

While the "style" element can also be used to include CSS directly within the HTML document, it is generally best for small amounts of CSS that are specific to that page. The "script" element is used for including JavaScript files, and the "meta" element is used to define metadata about the HTML document, such as character set or viewport settings. Thus, for linking CSS styles, the "link" element is the correct choice.