This is a lot easier if you don't support PowerPoint animations. A few years ago I used to work on a competing product and we did support PowerPoint animations. I thought about how to support animations in HTML and JavaScript quite a bit and it just became obvious that doing it in Flash was far easier.
Can't speak for PowerPoint, but my company does print to web to print again and our issues with HTML were always how precise it was in terms of rendering text and graphics:
Font rendering: With flash you can embed fonts and have them render pixel perfect across browsers. Now with WOFF fonts (see google fonts, font squirrel, typekit) and CSS3 you can pretty reliably embed custom fonts in HTML.
Graphics: Flash was first of all a vector animation tool so this was its strong suit for many years. Now you can use libs like raphealjs, d3js, and fabricjs to render vectors with svg/vml or canvas.
I'll note that SVG could do much of this years ago, however IE not supporting it until IE9 pretty much killed its progress. If SVG fixed multiline text (it uses tspans to break up lines), I think it would be a better solution than canvas for rendering graphics and rich documents in the browser.
Flash also has text spans for displaying blocks of text. It is not too hard to figure out the line-breaking if you were doing the rendering on the server - Java fonts and other text related code gives a pretty decent approximation to what the Flash Player does. However Flash's text fields which does the wrapping for you and has rudimentary HTML support is a lot easier to use.
This is pretty much what I ran into. That and we had a business requirement that it worked in IE 6. At that point Flash is going to be your best bet and even that has cross browser issues from time to time.
