By "hypertext" I mean non-sequential writing.
Ordinary writing is sequential for two reasons. First, it grew out of speech and speech-making, which have to be sequential; and second, because books are not convenient to read except in a sequence. But the structures of ideas are not sequential. They tie together every which-way. And when we write, we are always trying to tie things together in non-sequential ways (see nearby). The footnote is a break from sequence; but it cannot readily be extended (though some, like Will Cuppy, have toyed with the technique)....
See hypertext history and this collection of references on hypertext for more information.
(1) Ted Nelson, Computer Lib/Dream Machines, rev. ed. (Tempus Books of Microsoft Press, Redmond, Washington, 1987), p. 29.