Centering a DIV element

Trying to center a DIV element can be a frustrating experience.

<div id="content">
This is a DIV block that is to be centered. I don't
want the text to be centred, though, just the block.
</div>

The trick is to specify a width for the div, then set the left and right margins to auto.

#content {
  width: 700px ;
  margin-left: auto ;
  margin-right: auto ;
}

Just remember, this won't work if you haven't set a width for the