How to center an element in the middle of the browser window?

To do this you need to know the size of the element you are centering. Any measurement will do (i.e. px, em, percent), but it has to have a fixed size.

The css will look as follows:

 // Replace X and Y with a number and u with a unit. do calculations
 // and remove parens
.centered_div {
   width: Xu;
   height: Yu;
   position: absolute;
   top: 50%;
   left: 50%;
   margin-left: -(X/2)u;
   margin-top: -(Y/2)u;
}

Edit: This centers in the viewport. You can only center in the browser window using JavaScript. But that might be good enough anyway, since you probably want to display a popup/modal box?

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