It is a feature of CSS 2.1. See my reply to chubs above. CSS 2.1 10.6.4 requires that "margin: auto" on both sides of an absolutely-positioned element mean "vertically center it".
Sure, and we have that now, with flexbox. "align-items" is the property you're looking for.
I find it hard to fault the designers of CSS 2.1 for not being more complex than it was at the time. CSS 2 was a huge advance over CSS 1, so much so that a common feeling at the time was that CSS 2 had so many features no browser vendor would ever implement it all. The authors of CSS 2.1 had to maintain a delicate balance between giving authors the tools they needed and exploding the complexity of the spec, and making vertical alignment fall out of "margin: auto" was one of the ways they did just that.
You need absolute positioning because margins in the normal flow are special in that they're subject to margin collapse, and trying to specify some sort of special interaction between margin collapse and "margin: auto" would be too complex. Absolute positioning is also nice because it has a well-defined notion of a containing block, which allows CSS 2.1 to easily define "what I'm vertically centering relative to".
The way absolute centering vertical alignment works in CSS makes sense once you understand containing blocks: you specify "position: absolute" to disable margin collapse and establish a containing block, you specify the offsets from the edges of the containing block that you want, and then you invoke the vertical-centering feature in CSS by saying "margin: auto".
If I wanted to do those things, I should write "margin-collapse: disabled" and "vertical-align: center". Making your abstractions leaky on purpose doesn't mean you're justified in doing it.
I don't understand CSS all that well, but I do understand basic layout concepts and my specific intent, and it isn't very productive to have to wait around for people like you to write books and answer stack overflow questions just so I can use CSS to do what it was designed to do. If I write "margin: auto" on an absolutely positioned element, I would expect it to do nothing, since those are just contradictory demands. I wouldn't expect it to do something completely unrelated like vertical centering.
> If I wanted to do those things, I should write "margin-collapse: disabled" and "vertical-align: center". Making your abstractions leaky on purpose doesn't mean you're justified in doing it.
Adding "margin-collapse: disabled" would increase the complexity of CSS for no real reason, because now there would be three modes instead of two: static block, static-block-with-margin-collapse-disabled, and absolute position. There would be no benefit other than increased complexity of what is already a quite complex spec.
The real problem, IMHO, is that margin collapse exists at all. But that is not a CSS problem: that is a problem that goes way back to ancient, pre-CSS versions of HTML. A design goal of CSS, which was vital for its success, was to be backwards-compatible with HTML, in that you can describe every HTML layout in terms of CSS. (This is still not fully realized, because there are some bizarre corner cases involving things like <center>, which for example Hacker News relies on, but it's mostly there.)
