this is a concept in electronic publishing, more specifically in the CSS2 specification. The W3C says:
In some cases, authors may want user agents to render content that does not come from the document tree.
One familiar example of this is a numbered list; the author does not want to list the numbers explicitly, he or she wants the user agent to generate them automatically. Similarly, authors may want the user agent to insert the word "Figure" before the caption of a figure, or "Chapter 7" before the seventh chapter title. For audio or braille in particular, user agents should be able to insert these strings.
So, for example, suppose you make a paragraph class for notes, and today you decide that you want every note to be preceded by NB, in red italics:

P.note:before {content: "NB: "; color: red; font-style: italic}

Notice the use of the before pseudo-element. CSS2 can also generate counters, quote marks and other clever things.

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