Forget HTML5 vs Flash; what really matters is interoperability, and being viewable by everyone.
I don't have Internet Explorer (at least not until I get Civ5 and install Windows :S ). I use Chrome as my main browser, and Safari as my backup browser.
Thankfully the page worked in Safari... But what if it didn't? What if it only worked in IE, for some odd reason? I wouldn't be able to view the page...
Cutting out people based on their browsers is soooo 90s... Flash or no flash, HTML5 or HTML4, what matters is being able to view it.
HTML5 is a red herring. Most people, when referring to it, just refer to the video portion of it, or a few fancy animations or whatever. HTML as a whole is much more important, heck it's even used as a platform to launch Flash objects lol.
I think the big difference between Flash and HTML (nevermind the iphone/ipad's lack of Flash) is that, when the Flash applet has a bug, it just doesn't show up; when HTML has a bug, the browser shows as much as it can show ("graceful degradation" is a core principle of HTML rendering).
This is probably why so many people hate Flash, myself included. I love what they can do with it, but as soon as there's any issue, you're SOL and are left cursing at the site or the developers or who knows what else.
Regardless... About the page itself.. I wonder why not all the leaders have the animation?

Maybe to keep some surprises for the game?
N