This is correct. Mod parent up. No disrespect to HTML5, but it is not going to play any key role for "authorship," (which is, although beside the point, absolutely the incorrect term for the query; "publishing and distribution" is what is meant and what should have been used).
Any writing will be composed however the author feels most comfortable or creative, via pen and paper, dictation, typewriter, or word processor. They're NOT going to compose and tag HTML code for crissakes! I realize some of you supernerds have already, good for you, but no one cares. Write with the method you want to write with, Napoleon. GOSH!
Any written work, or book of images, or any combination of the medium, that is ever intended to be physically printed en mass, is going to be normalized as PDF no matter what the original form of the composition, even if it is a book of mathematics that is typeset in LaTeX, even if it is a small run of physical books and the majority of the tokens are sold as eBooks. If it is going to press, it will be PDF at some point. PDF is fine for digital distribution, but it is not ideal for eReaders, per se, unless the digital consumer desires a digital representation to be identical content to the printed book, in which case PDF is ideal.
IMO, there will be no single winner of the digital formats, because there's plenty of room for all of them, and then some. It's digital... who cares? Let's give the consumer some choice, it will make no significant difference in cost. Publish your book, print and sell the hardbound and softbound editions, and make available PDF, ePub, and Kindle format, Plaintext, DjVu, CHM, HTML, and sure, whiz bang HTML5 with JavaScript and video if you want... and any other versions for the digital consumer. There's no problem. Stop evangelizing the need for a solution to a problem that doesn't exist.