There's been a lot of buzz in the short few hours since Google's latest Doodle showed up on their home page on September 7th, 2010. One key buzzword which has been mentioned a lot is "HTML5", the internet's favorite New Thing™. While Google has yet to release an official statement regarding what the significance of this doodle is, one thing seems evident: it's not about HTML5.
Click to view full-size, or inspect it yourself! Chrome's built-in HTML inspector works, too.
It does appear that Google is loading different scripts for different browsers. The doodle still works fine in IE8, which supports neither HTML5 nor CSS3 rounded corners. It would appear that in IE8, the DIVs have a background image of a circle set, rather than using rounded corners, which leads to a slightly choppier, less pretty, but still fun to play with experience. And as for IE6 users? They'll never even know this was happening — all they get is the standard logo.
So, does this answer the question of what Google's intent is with this doodle? Nope. Does it make it any less awesome to play with? Nope! I'm sure millions of users all over the world will happily waste many hours today waving their mice around the Google home page, and most of them won't care whether it uses HTML5 or not. But just for the record: this isn't an HTML5 demo, or if it is, they're not using it in such a way that meets the eye.
Note to any Google engineers who happen upon this and disagree: if you did use some magnificent HTML5 feature and I just was too dense to realize, I apologize. It would be wonderful to hear from you about what I missed. It's entirely possible that I'm wrong; after all, who am I to argue with Google?
Update: It has been suggested that this may be a birthday celebration for Google. This seems like a reasonable hypothesis, given that Google was incorporated on September 4th, which was just a few days ago.