Complex sentence: Van Dam is perhaps most renowned for building the first hypertext system, HES in the late 1960s, and with it and its immediate successor FRESS, pioneering the use of hypertext in the humanities and in pedagogy.
Split into simpler sentences: Van Dam is perhaps most known for building the first hypertext system, HES, in the late 1960s. With it and its immediate successor, FRESS, he was an early proponent of the use of hypertext in the humanities and in pedagogy.