HTML publishing

Does online learning content have to be created in HTML?

What are the advantages over other formats such as PDF? And do these advantages outweigh the benefits of being able to rapidly author in whatever software you like and convert to PDF?

Using a LCMS enables content packages with navigation
Hyperlinking can still be included to external sites
Formatting can be created in stylesheets, and are more accessible than CSS
File sizes are becoming negligible in a ubiquitous high bandwidth environment

Asking subject matter experts to invest in learning HTML is distracting them from focussing on their subject to creating content. Better for them to apply design on building the interaction with that content than on the creation of it.

The big downfall - accessibility
Can PDF be made accessible to W3C guidelines?
http://www.adobe.com/products/acrobat/access_booklet.html