This post was originally published on my blog.
If you're unlucky enough to have to support non-flexbox-capable browsers,
I'm pretty sure you had nightmares about vertical alignment. It's the most
ridiculously complicated thing in CSS. There are urban legends about people,
who once aligned a box vertically in the middle of the screen, but they have
never been seen again.
Usually, the solution is the following: use absolute positioning, top: 50%,
and use negative margins to set the size of the element. If you can calc(),
even better.
However, this only works if you know the height of the element beforehand.
This is not always the case: you might want to have a dialog which grows with
the content. And it's counter-intuitive as well: negative margins just don't make sense.
We do have something though that looks like exactly what we need. It's called
vertical-align: middle, the first disappointment in every web developers life.
It seems to be the obvious solution - but it just doesn't work.
The reason is that vertical-align is for text formatting. It doesn't set the alignment
of the children of the element, like text-align, but aligns the children themselves,
relative to each other. This is a very common issue
in HTML: It is designed around flowing text, not UI, so it feels unnatural
to do things with it which should be obvious in a UI development framework.
That doesn't mean we cannot use vertical-align, we just need to be a bit creative.
Let's say we have a .container element, and a .box. As we already know, it should be applied
to .box, not .container, but it still doesn't seem to be doing anything. The reason being,
since it is the only child, there's nobody to align to. We need to add another one as a reference.
We can use the ::beforepseudoelement. This will be a ruler which we can align our .boxagainst.
We use an inline-block display for both of our elements, so
we can align them nicely. Our 'ruler' should have a 100% height, so it's like we were aligning
to the container's. Width should be 0, to hide it. And voilà, our box is vertically aligned!
Check out the working example (borders added for demonstration) here.