Centering vertically and horizontally without worrying about height or width

The following technique allows you to add your content to an HTML element and center it both horizontally and vertically without worrying about its height or width.

The outer container

• should have display: table;

The inner container

• should have display: table-cell;
• should have vertical-align: middle;
• should have text-align: center;

The content box

• should have display: inline-block;
• should re-adjust the horizontal text-alignment to eg. text-align: left; or text-align: right;, unless you want text to be centered

Demo

HTML

<div class="outer-container">
   <div class="inner-container">
     <div class="centered-content">
        You can put anything here!
     </div>
   </div>
</div>

CSS

body {
    margin : 0;
}

.outer-container {
    position : absolute;
    display: table;
    width: 100%; /* This could be ANY width */
    height: 100%; /* This could be ANY height */
    background: #ccc;
}

.inner-container {
    display: table-cell;
    vertical-align: middle;
    text-align: center;
}

.centered-content {
    display: inline-block;
    text-align: left;
    background: #fff;
    padding: 20px;
    border: 1px solid #000;
}

See also this Fiddle!