XML-Based Languages

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• mathematics (MathML): <plus/>, <times/>, <power/>, <mrow>, <msup>, <mi>, <mn>
• phone books (roamPhoneBook): <telephoneNumber>, <userNamePrefix>, <address>
• frequently asked questions (QAML): <title>, <content>, <email>
• real estate (RELML): <num-beds>, <price>, <taxes>
• automotive, banking, chemistry, ...

DTD (Data Type Definition): a formalism for specifying constraints on XML-based languages.

There are tools for validating XML documents against DTD’s.