What is an advantage of using external CSS?

Choosing to use external CSS provides the significant advantage of allowing multiple HTML files to share the same style. By linking to a single CSS file, different pages can maintain a consistent look and feel without the need to duplicate styling rules in each HTML document. This not only saves time during the development process but also makes it easier to update styles across an entire website. If a change is required, such as updating a color scheme or modifying a layout, adjustments can be made in one central location, instantly reflecting across all linked HTML files. This modular approach enhances maintainability and reduces potential inconsistencies in design.

The other options, while related to CSS, do not capture the primary benefit of using external stylesheets. For instance, although external CSS can help in keeping the overall size of HTML documents smaller, the main advantage lies in the shared stylesheet feature.