The colour choices in the image with caption beginning "Jeffrey Zeldman's homepage, March 1997" are hard on my eyes. However, the point might have been to show folks how to exert control over colours and fonts, as opposed to actually communicating. The 90s were quite a different thing than whatever we call the present decade.
A big annoyance of the early web was all the stupid blinking text and pointless little animations. Luckily we've moved past them. Of course, today it's all about ads, which is the tip of a spear that is quite unpleasant.
Plus ça change.
While black on yellow isn't necessarily the best combination, you have to take into account the medium of the times.
On the typical 15" CRT with its high dot pitch and relatively dark tube that garish yellow would be far less garish. The black, by virtue of the display, would also not be as black and more a very deep grey. The text contrast would also be somewhat comfortable as it would get an anti-aliased effect even though the OS at the time didn't have good font smoothing at the time.
It was all about ads back then too, they just weren't so targeted.
I don't think we can judge a screenshot of Zeldman's site using today's displays. You are not now seeing what people saw then.