Function of Markup Languages
Markup languages are designed to instruct computers how to process data. The term markup derives from early print publishers, who would "mark up" text by hand to indicate to the printer which front size to use where, n which weight, using what form of alignment and so forth. In other words, the earliest markup languages were dedicated to passing formatting instructions.
Markup instructions are generally referred to as tags, and the process of marking up a document is sometimes called tagging. Early word-processing programs required the user to perform manual tagging. Today, most tagging is performed transparently in programs, and usually takes place using a proprietary system. The different methods for tagging text made it difficult for people to exchange data with each other. With the advent of Internet, it became more valuable and more imperative for authors to be able to interchange documents in a single format that was easy to use, yet powerful and aesthetically acceptable.
Markup specifically designed to affect the appearance of a document is commonly called presentation markup or procedural markup because it instructs the computer how to render the text. However, organizations that process huge numbers of documents, such as government and bureaucratic entities, quickly found that it was more important to know what the data represented rather than how it looked. Markup was then created to describe the content of the page. This type of markup is called logical or structural markup. Structural markup is also called descriptive or generic markup.
The following is HTML presentational or procedural markup for tagging the word "Summary" to appear in bold print:
Summary
In logical or structural markup, it makes more sense to designate the word "Summary" as a section header, which describes not just its appearance but also its context in the document. The following is HTML structural markup for tagging the word "Summary" as a level-one heading:
Summary
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