This is how he defines it:
A web-essay is a way of thinking by playing with images and making networked connections. Mostly those connections are made by the way images are placed on the screen and then glued together with some words.(...)
Writing from the top to the bottom of writing tablets fits with a linear line of argumentation. Where people don't write they don't often do that; they tell stories in a chronological storyline to achieve suspense in their listeners; or they try to persuade their listeners by getting them excited and worked up.The whole essay deserves much more time than I could give it this morning, it is a fascinating read to which I will come back again and again.