There's something to this -- but not much. I say this as somebody whio has been writing for the web since -- well, since there were no other markets for low level freelancers that paid a decent rate.

Really there are just two rules -- although the first one is pretty much a confirmation of the article: the opening has to be a real grabber. If your article is in a magazine, people can only go on to the next item, if it's on the web, they can go anywhere. People may be willing to read 100 pages of a boring book before they put it down, and one page of a boring print article, but on the web, moving to something else is too easy. You need that first line.

Two screens is the effective limit to the reader's patience.