CSS Vertical Alignment:

div {
display:flex;
align-items:center;
}

This is not a problem anymore, so can we stop pretending like it is?

@thmnmlst always worked for me

I'd say it's still a problem for some due to its poor support on older browsers. Some of us here are still required to support <IE9 (and yes, it sucks).

And did you know about
position: absolute
top: 50%
left: 50%
transform: translateX(-50%) translateY(-50%)
?

It was never a problem - theoretically. But shit always fucks up when stuff gets a bit bigger.

EDIT: and old browsers suck ofc.

For the flexbox to work properly, the container need be full height as well, otherwise the children are vertically centered, but not over the full height of the page.

Look, I've built a rocket during the time it took you to center that vertically ;)