I'd really like to see something like this and have thought that an online publishing engine which would produce HTML, ePub, PDF, or other output formats or endpoints on demand might be useful.

There are sites already which provide some form of this, notably sites using MediaWiki (which notably powers Wikipedia and Wikisource) offer multiple download options, though I don't think it's quite as flexible as I'd prefer.

Client support would be the other factor, with current Web browsers not being particularly multi-format. Though to be fair, Firefox, Safari, and Chrome all now natively support PDF documents.

I suspect that a render-to-ePub would be a much better alternative to standard HTML for mobile devices. ePub is inherently fluid, and tends to offer a subset of full HTML capabilities. (ePub is of course an archive of HTML, stylesheet, and other elements, though usually it's more strongly textually oriented than the modern Web is.)

For sufficiently large tablets, PDFs are actually a quite good document format.