We are so used to the serial ordering of material in a textbook that we rarely give thought to the problem of finding a particular example or theorem or whatever. We remember the approximate location of the material and can easily find it. With web pages it is much more difficult. The material is not presented serially. We are faced with the equivalent of trying to find something in a collection of books, and we're not even sure which book to look through.
Students have trouble finding specific items, and so do the authors.
Topography is of paramount concern, and we have not yet satisfactorily resolved this issue. Entertaining and informative graphics. For example, at the end of a web page, we may have hot buttons that are links to some additional examples. If you go to one of these examples you can link to any of the additional examples, or you can return to the originating page. You cannot link to further material directly.