That makes sense about it being an inline element, I never really even noticed that.
It appears that an iframe is a replaced inline element as well since it is able to take dimensions, similar to an image.
I can't seem to find a detailed list of replaced inline elements.
Would you happen to know where one might be found?
The SP reference makes note of a few of them but does not go into detail with a thorough list.
In this test code below I can set dimensions on the iframe which seem to confirm it as a replaced element.
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
padding:0 0 20px;
<font color='Blue'>text-align:center;/*center the iframe*/</font>
<font color='Blue'> width:50%;
<h2>Main wrapping div with text-align:center</h2>
<iframe name="myFrame" src="http://www.rayswoodworks.com/css-demos.html" frameborder="0"></iframe>