Final answer:
A valid XML document must be well-formed and validate against a defined schema or DTD. It should follow syntax rules and adhere to the specified structure and data types.
Step-by-step explanation:
Requirements for a Valid XML Document
An XML document is considered valid if it meets two key requirements. First, it must be well-formed, meaning that it follows the generic syntax rules set by XML standards. These include having a single root element, properly nested tags, and correct use of start and end tags. Second, the XML document must validate against a defined schema or Document Type Definition (DTD), which specifies the structure and type of data allowed. If the XML document adheres to the schema or DTD, it is considered to be both well-formed and valid.
Examples of XML Document Validation
Let's consider an XML file that lists books. According to its DTD or schema, each book element may contain title, author, and genre elements, all of which need to be present for the document to be valid. If a book element is missing an author element, the document would not validate against its schema or DTD and hence would not be considered a valid XML document.