What are HTML5 presentations exactly... and what is the technology that they rely on?
Is it just plain CSS + AJAX? Or is it something else entirely?
An HTML5 presentation system is just a presentation framework written with HTML5 technologies. For example, some presentation systems: http://www.sitepoint.com/5-free-html5-presentation-systems/.
Because presentation slides are dynamic, frameworks are necessarily written in Javascript and CSS.
Basically, it's just the same as all other client-side frameworks built on top of Javascript and CSS. The only difference is the purpose that it is designed for. Where other frameworks are made for dynamic client-side web applications, HTML5 presentation systems are frameworks designed for ... presentations.