No, I've been playing around with this and pouring over all the pages of my sites, and I've never done this either apparently.
When you use the width: 6em; style rule, it creates a magenta box 6em long. But depending on the text you have in the XHTML file, this may or may not be enough for the text. It may force the text to wrap onto a second line. If you remove the width rule, you get the heading onto one line, but the magenta box instantly snaps to the width of the screen (adjusting only for the margins). I've played with the margins and using percents and other units for the width, but the same basic problem remains. It sounds like you want the box to automatically expand to fit the length of the content. So far I haven't been able to find a solution.
...Depending on how many people you want to piss off, you could create a one cell table. I believe that would display the behavior you're looking for. ;) (Just a bad joke, and besides I'm not sure. It's been years since I did a table based layout.)
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