Yeah, most browsers also accept <hr style="text-align: center" />, but it's not standard CSS/XHTML. Using the auto margins is the correct way, but as Jellby pointed out, ADE doesn't seem to support that.
To create a separator with asterisks that has space above and below, you could do <p style="margin: 1.0em 0;">* * *</p>, which would give you the spacing without needing <p> </p>
I had reasons for wanting to use a fixed width, but since it's not supported, I'm not going to worry about it too much.