Vertically centering a div inside another div

tl;dr

Vertical align middle works, but you will have to use table-cell on your parent element and inline-block on the child.

This solution is not going to work in IE6 & 7.
Yours is the safer way to go for those.
But since you tagged your question with CSS3 and HTML5 I was thinking that you don't mind using a modern solution.

The classic solution (table layout)

This was my original answer. It still works fine and is the solution with the widest support. Table-layout will impact your rendering performance so I would suggest that you use one of the more modern solutions.

Here is an example

Tested in:

• FF3.5+
• FF4+
• Safari 5+
• Chrome 11+
• IE9+

HTML

<div class="cn"><div class="inner">your content</div></div>

CSS

.cn {
  display: table-cell;
  width: 500px;
  height: 500px;
  vertical-align: middle;
  text-align: center;
}

.inner {
  display: inline-block;
  width: 200px; height: 200px;
}

Modern solution (transform)

Since transforms are fairly well supported now there is an easier way to do it.

CSS

.cn {
  position: relative;
  width: 500px;
  height: 500px;
}

.inner {
  position: absolute;
  top: 50%; left: 50%;
  transform: translate(-50%,-50%);
  width: 200px;
  height: 200px;
}

Demo

♥ my favourite modern solution (flexbox)

I started to use flexbox more and more its also well supported now Its by far the easiest way.

CSS

.cn {
  display: flex;
  justify-content: center;
  align-items: center; 
}

Demo

More examples & possibilities: Compare all the methods on one pages

Another way of achieving this horizontal and vertical centering is:

.Absolute-Center {
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

(Reference)

Vertical Align Anything with just 3 lines of CSS

HTML

<div class="parent-of-element">

    <div class="element">
        <p>Hello</p>
    </div>

</div>

Simplest

.element {
  position: relative;
  top: 50%;
  transform: translateY(-50%);
}

CSS

.parent-of-element {
   position: relative;
   height: 500px;
   /* or height: 73.61% */
   /* or height: 35vh */
   /* or height: ANY HEIGHT */
}

.element {
  position: absolute;
  top: 50%;

  -webkit-transform: translateY(-50%);
      -ms-transform: translateY(-50%);
          transform: translateY(-50%);
}

According to shouldiprefix this are the only prefixes you need

You can also use % as the value for the 'height' property of .parent-of-element, as long as parent of element has height or some content that expands its vertical size.

Another way is using Transform Translate

Outer Div must set its position to relative or fixed, and the Inner Div must set its position to absolute, top and left to 50% and apply a transform: translate(-50%, -50%).

div.cn {
    position: relative;
    width: 200px;
    height: 200px;
    background: gray;
    text-align: center;
}

div.inner {
    position: absolute;
    top: 50%;
    left: 50%;
    width: 100px;
    height: 100px;
    -webkit-transform: translate(-50%, -50%);  
    transform: translate(-50%, -50%);   
    background: red;
  
}

<div class="cn">
    <div class="inner">
        test
    </div>
</div>

Tested in:

• Opera 24.0 (minimum 12.1)
• Safari 5.1.7 (minimum 4 with -webkit- prefix)
• Firefox 31.0 (minimum 3.6 with -moz- prefix, from 16 without prefix)
• Chrome 36 (minimum 11 with -webkit- prefix, from 36 without prefix)
• IE 11, 10 (minimum 9 with -ms- prefix, from 10 without prefix)
• More browsers, Can I Use?