Well, Pages can do that. Of course, not for complicated things. But the HTML export in Pages works well. The only thing you have to do is to set a new page attribute through the "Page Setup" menu item. Choose Custom Paper size, and put, for instance, height = 400 cm, width = 30 cm, and maybe 1.5 cm for left margin, and that's it.
[robg adds: Pages' HTML export feature has been much-maligned; it makes a shambles out of most of the complex templates in Pages, as it can't handle the placement of the images. But if you've got a basic text document, you'll get a good looking web page out of it. The only caveat I would offer is that it's not really efficient HTML -- if you view the source, you'll probably find a

