I use pdf files, generated with Beamer/LaTeX, and whatever pdf viewer is available on the computer I am presenting on. While I like the Beamer/LaTeX/PDF combination better than PowerPoint type applications I am not particularly happy with its ability to embed media (sounds and videos). Further, common PDF viewers (e.g., Acrobat, Evince, and Okular) do not provide any type of "presenter" view with a clock and notes on one screen and the slides on a projector. The presenter view in advanced viewers like impressive are pretty limited. I am thinking of switching to an HTML5 based system (possibly reveal.js), but wanted to know what the drawbacks of HTML5 based presentations are.
From a web developer perspective, the only issue I can see is compatibility. If the provided computer does not provide an HTML5-ready browser, the presentation may not work. Unfortunately, having been in labs and lectures with institution-provided computers, there is no guarantee that an IT department has provided the newest browser. While I'd like to think that people, especially at institutions, are keeping their machines up to date, we have to assume that some machines have been forgotten or have not been maintained.
This HTML5 readiness graph demonstrates that IE8, which still has a huge market share, is woefully prepared for any HTML5 feature. IE9 has some functionality over IE8 and appears to cover the audio/video portions you require, but only IE10 appears to be HTML5-ready as of last year.
I tried the reveal.js demo on IE8 just now. The rotating transitions don't work at all (it acts like Powerpoint basically), but it was usable. Since there are no videos or audio, I can't test those, but a standard presentation would at least be doable even in that browser.
It is unlikely that any IE9- browsers will ever reach full HTML5 support due to the security risks and time drain required, so any attempt to present on an IE8 browser is likely to be bare-bones and equivalent to viewing a PowerPoint.
However, this can be largely mitigated with the use of a PortableApp web browser such as Portable Google Chrome. A small footprint that can be carried around in a flash drive with the presentation.
From a student perspective,
reveal.js might appear to be more confusing if you share the files, as it probably isn't just a
foo.ppt file, but a source document, the JS, and so on. You will probably need to provide a read-me for being able to view the presentation. I'm sure everyone here knows how to open and view a PowerPoint, but some of us might stumble a bit with a more complex solution that may depend on multiple files and folders. That, and the student would need an HTML5 browser to get all the features working.
You probably will also need to direct PDF copy of the slides because I didn't see any noticeable way to print the presentation unless the stylesheet natively does that from the print menu.
Overall HTML5 Presentations are great, but that's not to say that they'll be better than the alternatives in every respect, or that you'll never have a bad time.
One challenge I've experienced is when I need to collaborate with colleagues on the presentation -- many lay people are intimidated by HTML or Markdown-based tools. There aren't a lot of cloud collaboration apps that will let you upload plain HTML/Markdown and then let your friends add, edit, or comment on your work. You may end up having to do a lot of converting back and forth between HTML and Office formats if you work with non-techie collaborators.
Similarly, people are often confused about how to open an HTML presentation when you send them an index.html file and a folder full of assets. You can remedy this by hosting things yourself in a Dropbox folder, but that isn't always awesome.
I use Reveal.js often, but I think it's fair to say that I spend more time developing an HTML presentation than I would in Apple Keynote. I usually write out my ideas in Markdown first, then convert that to HTML and edit it further within the Reveal.js codeset (from my code editor).
If your presentations are mostly made up of text and un-altered images, Markdown/HTML can be very fast. If you like to futz with the colors, fonts, and modify your images, that's faster in a presentation software like Keynote. Even simple things like cropping images, writing over images, etc. would require you to do them in an external graphics editor before incorporating them into your HTML presentation.
So basically the biggest problems with HTML presentations are working with other people and focusing too much on the visuals. Other than that, it's great.