How to design a CSS for a centered floating confirm dialog?

CSS3 transform can be used to avoid hardcoded width and margins:

.dialog {
  top: 50%;
  left: 50%;
  -webkit-transform: translateX(-50%) translateY(-50%);
  -moz-transform: translateX(-50%) translateY(-50%);
  -ms-transform: translateX(-50%) translateY(-50%);
  transform: translateX(-50%) translateY(-50%);
}

Example usage: http://tympanus.net/codrops/2013/06/25/nifty-modal-window-effects/

Compared to @Toma's answer, you can do it on one element

#theDiv {
    position: fixed;
    top: 50%;
    left: 50%;
    width: 200px;
    margin-left: -100px;
    height: 50px;
    margin-top: -25px;
    background: yellow;
}

<div id="theDiv" />

This allows you to place it anywhere on the page regardless of parent containers and their position properties.

Try using this CSS

#centerpoint {
    top: 50%;
    left: 50%;
    position: absolute;
}

#dialog {
    position: relative;

    width: 600px;
    margin-left: -300px;

    height: 400px;
    margin-top: -200px;
}

<div id="centerpoint">
    <div id="dialog"></div>
</div>

#centerpoint should be the container div of the dialog

Note that the #centerpoint DIV should be inside the body element or inside elements that don't have a position: relative; property