I found that with the page saved simply as windows ASCII/ANSI then the Ú did not show in some browsers and IE6 really messed up, displaying /em> (part of the source) after the extended character!
I think the subscript is being pushed out of the <body> and the <body> is not extending to accommodate? Could try setting a height on the body or setting overflow? Just a thought.
I've not tried this in IE7. Is the underline the same as that for IE6, or is IE6 unique in this respect. The non-continous underline strikes me as a significant difference.
But I think we have to keep in mind that HTML/CSS is not intended for exact typographic layout. The browsers are entitled to render things slightly differently as there is no exact specification in this area. In this case I think the browsers are still just about within the bounds of reasonable interpretation. Or are they?
There might be other systems available, depending on your purpose, if you need more exact typographic layout?