Document Type Definition - DTD - GeeksforGeeks
https://www.geeksforgeeks.org/document-type-definition-dtd/
WebLast Updated : 06 Nov, 2020. A Document Type Definition (DTD) describes the tree structure of a document and something about its data. It is a set of markup affirmations that actually define a type of document for the SGML family, like GML, SGML, HTML, XML.
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