DTD - Element Declaration (<!ELEMENT>) in DTD

Element declaration is a fundamental part of DTD and is used to define the content and structure of an XML element. It specifies what an element can contain and how it should be used within an XML document.

The <!ELEMENT> declaration tells the XML parser whether an element contains text, child elements, or no content at all. This helps control the type of data stored inside the element and avoids incorrect usage.

DTD uses element declarations to define the relationships between elements. It specifies which elements can be nested inside other elements and in what order they must appear. This ensures a logical document structure.

Element declaration also defines content models, such as EMPTY elements, elements containing parsed character data (#PCDATA), or elements containing other elements. These content models are essential for validation.

Another important role of <!ELEMENT> is enforcing sequence and occurrence rules. DTD can specify whether an element must appear once, multiple times, or is optional using symbols like ?, *, and +.

During validation, the XML parser checks every element against its declaration. If an element contains invalid content or is used incorrectly, the document fails validation.

Thus, element declaration ensures consistency, correctness, and reliability of XML documents. It acts as a rule set that guarantees proper element usage and well-structured XML data.