The third principle is to avoid e-learning with extraneous words. Limited screen space would require concise wording. To much narration can add to the file size which could result in technical problems.
The same reasons extra graphics or audio can be a hindrance, also apply to text. It it recommended to avoid extra text for interest, elaboration and technical explanation.
In the Photography text book by Ben Long, Text and images that would be considered extraneous and contained in these grey boxes with black headers. They provide more detail about the subject matter of the section, but can be avoided without causing a disruption. Obviously the information is relevant enough to still be worth the printing.
ref: e-Learning and the Science of Instruction; Ruth Colvin Clark • Richard E. Mayer