DTD Elements and Attributes

Estimated study time: 16 minutes.

A DTD defines both the elements a document can use and the attributes those elements are allowed to carry. Each has its own declaration syntax.

Declaring Elements

<!ELEMENT course (title, duration, modules)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT duration (#PCDATA)>
<!ELEMENT modules (module+)>
<!ELEMENT module (#PCDATA)>

Common content model symbols:

• #PCDATA — parsed character (text) data
• , — elements must appear in this exact order
• + — one or more occurrences
• * — zero or more occurrences
• ? — optional, zero or one occurrence

Declaring Attributes

<!ATTLIST course
  id CDATA #REQUIRED
  level (beginner|intermediate|advanced) "beginner">

This says the course element must have an id attribute (required, free text), and may have a level attribute restricted to one of three values, defaulting to "beginner" if omitted.

Common Attribute Defaults

• #REQUIRED — the attribute must always be present.
• #IMPLIED — the attribute is optional.
• #FIXED "value" — the attribute always has this exact value.
• "default" — a specific default value used when the attribute is omitted.