CSS how to position an element in a middle (half height / vertical 50%) of another element

This is pretty late but I tried to make an example on codepen which shows a working example if I understood your question correctly.

http://codepen.io/trgraglia/pen/RWbKyj

Use CSS transform.

For example, transform:translateX(-50%);, will move an element left by 50% of ITSELF. Now you can align it to the parent with position and bottom or top or left or right and then move it by the dimensions of itself.

Hope this helps!

using CSS & jQuery-

CSS

.div{
    top:50%
}

jQuery

var divheight = $(.div).height() / 2;
$(.div).attr('style','margin-top:-'+divheight+'px;');

If you don't know both heights, there are only two ways:

1. Set the parent display: table and children display: table-cell and vertical-align: middle.
2. Using CSS3 Flexbox: http://www.w3.org/TR/css3-flexbox/ but with limited browser support: http://caniuse.com/#search=flexbox

If you are interested in this topic, here there is a good article with more tips: http://www.vanseodesign.com/css/vertical-centering/

As you can see, the only way to vertical align an element without knowing its height and nor using CSS3 is using display: table-cell.