What does the declaration of a document type define?

The declaration of a document type, often referred to as the DOCTYPE, serves to indicate the type of document being used, specifically in HTML and XHTML. It informs the web browser about the version of the markup language that is being utilized to render the content correctly. This is essential for ensuring that browsers correctly interpret the elements and structure within the document according to the specified standard.

By declaring the document type appropriately, developers can leverage the full capabilities of web technologies while keeping the content accessible and rendering consistently across different browsers. This declaration does not define the structure of the content itself or the programming language used, nor does it guarantee compatibility with specific browser versions, but rather establishes a baseline for understanding the document's intended format and version.