The great thing is that with one (X)HTML document, you can have a slide show, a printer-friendly version, and a screen presentation. I put markup and CSS examples right there in the document, ready to be printed, and simply hide them in the slide show. In some cases, I printed out the file for handouts, and then used the exact same file for the slide show. It was very, very handy. Eric's Archived Thoughts: Slide Show Beta
Dear Apple/Steve: please make Keynote export to this "format" automatically.