CSS: Vertical centering in a position:absolute div

Question

I have this code:

<div class="parent">
    <div class="child">something</div>
</div>

The parent should be 200px width and height, and the child 100px width and height. The parent is also set to position:absolute. Is it possible to horizontaly and verticaly center the child relative to the parent and how?

The result: If you set black background to the parent and white to the child, the whole thing should look like there is a white square with a large black border.

Static margins and paddings are out of the question since the size of the child will change dynamically.

---

That might not be the proper solution, but what I am actually trying to achieve is a black square which I can change the size of with jQuery and it will stay at the same absolute position, like it is being scaled from the center, not from the top left corner.

I thought I could set the position of a parent and let the child center horizontaly and verticaly auto while changing it's size, so I would get the proper 'scale'-like effect.

Answer 1

I used jQuery UI position API to position the child div at center and then on .animate based on border-width you should change the top and left accordingly.

Try out the DEMO to understand the below code DEMO

After Animation:

HTML:

<div class="parent">
    <div class="child">something</div>
</div>

<button>Animate</button>

JS:

$(document).ready (function () {
    positionChild();
});


function positionChild() {
    $( ".child" ).position({
                of: $( ".parent" ),
                my: "center center",
                at: "center center"
    });
}

$('button').click (function () {
    $('.child').animate({'border-width': 47, top: '-=46', left: '-=46'}, 1000);
});

CSS:

.parent { 
    position: absolute;  
    width: 200px; 
    height: 200px; 
    background-color: black; 
    top: 100px; 
    left: 100px; 
}

.child { 
    width: 100px; 
    height: 100px; 
    background-color: #c2c2c2; 
    border: 1px solid blue;
}