What is the primary purpose of a Document Type Definition (DTD)?

The primary purpose of a Document Type Definition (DTD) is to provide a set of rules that an HTML document must conform to, ensuring that the document is properly structured and valid according to the specified markup language. DTDs serve as a guideline for both developers and browsers, outlining which elements, attributes, and entities are allowed in the document. By defining these rules, a DTD helps maintain consistency in the markup and assists browsers in rendering the content correctly.

While other options address various aspects of web development, they do not align with the specific function of a DTD. Defining structural rules of a website can be part of various aspects of web development, but it does not capture the essence of what a DTD does. Similarly, managing a database schema pertains to server-side applications rather than document structure, and enhancing page load speed is related to performance optimization rather than structural validation. Therefore, the correct answer reflects the specialized role of a DTD in ensuring the validity of HTML documents.