PowerPoint includes a lot of different options for saving presentations as web pages. Many of these options directly affect how compatible the resulting HTML will be with different browsers.
Download The Office 2000 Browser Compatibility white paper to learn what PPT features are supported by various browsers and how the different options affect compatibility.
Animate Your PowerPoint 2002 Web Presentation gives a concise outline of which animation effects will work well on the web and which won't.
Note that users who don't have Office 2002 or later installed will need to download and install the Office Animation Runtime (approximately 4mb) before they'll be able to view animated presentations in Internet Explorer 5 and later.
If you're looking for the ultimate in browser compatibility, visit The PPTools PPT2HTML page to learn about a completely different way to convert your presentations for the web.