SGML, like HTML, uses only standard ASCII characters and uses specialized tags to delimit language elements.
SGML differs from more specialized markup languages such as HTML in that it has a very small set of native operators. Instead of building element definitions into the language, they are defined as part of each document. The advantage of this approach is that files can be validated for consistency without knowing what additions and modifications have been made to the language since the validation code was written. Presentation software, however, still must be aware of language additions or changes and will ignore any operators it does not recognize.
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