I want to display a CSS horizontal menu in the middle of the `header` element and make sure that it stays in the middle when resizing the browser window. I of course tried `margin: auto;`, but it also did not work for me.

[jsfiddle code](http://jsfiddle.net/psot98/czqKV/4/embedded/result/)
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```
header .center {
  background: url(../images/header-center.png) no-repeat;
  width: 510px;
  height: 100%;
  /*margin: auto;*/
}

```