centering a div

It's easy to center text within a div. To center the actual div, I still find myself typing:

Clearly not the HTML 5 way. Is there a better simple way to do this? Everything I've seen is quite complex. I understand it will be more complex/longer than the original. However, being very long/complex feels like a deterrent to writing HTML 5 compliant code.

Set the left and right margins to auto, and give the <div> a width.

That works, but I don't understand why.

# shows it about a third in
div.center50 { margin: auto; width: 50% }

# shows it just short of center. maybe 40 percent in
div.center50 { margin: auto; width: 10% }

# shows it exactly centered
div.center50 { margin: auto; width: 0% }

The reason to do it in CSS is to keep presentation and content separate.

In the first rule you have, you're setting the width of the div to 50%, which means 50% of the width of the viewport. By setting margin to auto, you're saying, let the browser decide what the margin should be. In most browsers, the behavior is to set the margin so it's equal on both sides of the div.

I see. Thanks.