Structured document
A structured document is an electronic document where some method of embedded coding, such as mark-up, is used to give the whole, and parts, of the document various structural meanings according to a schema. A structured document whose mark-up doesn't break the schema and is designed to conform to and which obeys the syntax rules of its mark-up language is "well-formed".
The Standard Generalized Markup Language (SGML) has pioneered the concept of structured documents
As of 2009 the most widely used markup language, in all its evolving forms, is HTML, which is used to structure documents according to various Document Type Definition (DTD) schema defined and described by the W3C, which continually reviews, refines and evolves the specifications.
XML is the universal format for structured documents and data on the Web
In HTML a part of the logical structure of a document may be the document body; <body>, containing a first level heading; <h1>, and a paragraph; <p>.
<body>
</body>
One of the most attractive features of structured documents is that they can be reused in many contexts and presented in various ways on mobile phones, TV screens, speech synthesisers, and any other device which can be programmed to process them.
Other meaning can be ascribed to text which isn't structural. In the HTML fragment above, there is semantic markup which has nothing to do with structure; the first of these, the <strong> tag, means that the enclosed text should be given a strong emphasis. In visual terms this is equivalent to the bold, <b> tag, but in speech synthesisers this means a voice inflection giving strong emphasis is used. The term semantic markup excludes markup like the bold tag which has no meaning other than an instruction to a visual display. The strong tag means that the presentation of the enclosed text should have a strong emphasis in all presentation forms, not just visual.
The anchor <a> tag is a more obvious example of semantic markup unconcerned with structure, with its href attribute set it means that the text it surrounds is a hyperlink.
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