Historically, markup was used to refer to:

the process of marking manuscript copy for typesetting with directions for use of type fonts and sizes, spacing, indentation, etc. (from the Chicago Manual of Style, the bible of most publishers.)

Electronic Markup originally referred to the internal, sometimes invisible codes in documents which described the formatting.

In WYSIWYG systems, the system inserts the codes. In early WYSIWYG systems such as Wordstar, the markup is visible on the screen.

You can view the markup in this document by looking at the Source with whatever browser you are using.

The Style sheet (as in Word) is just a modernized version of markup.

Markup can be classified as one of two types:

Procedural Markup which is concerned with the appearance of text - its font, spacing etc.

Descriptive or Declarative Markup which is concerned with the structure or function of the tagged item.

Goldfarb believed that :

"markup should describe a document's structure and other attributes"

and should completely divorce structure from appearance while facilitating indexing, generation of selective views etc.

His ideas were ideal for leading the SGML design team.

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