[metaxy2]

I think the joke about it being "the only way to vertically center elements" is just referring to the fact that it's still kind of hard to do vertical centering and there's still no definitive, easy solution after 27 years of CSS. (It's now doable with Flexbox or CSS Grid as well as more obscure hacks, but it's still more fiddly than it should be.) Could be wrong, though.

Edit: I think thejohnconway has the answer [1]: the post was written before the Flexbox stuff for vertical centering was widely supported.

There were still less nasty options than 1x1 GIFs available, like "display: table-cell", so it's still very likely a joke.

[1] https://news.ycombinator.com/item?id=39127936

[neurostimulant]

I still need to consult a cheatsheet from time to time when working with flex.