What's the deal with the cake in T-Rex Runner (Downasaur)?

It marks the tenth anniversary of the release of the browser 10 years ago.

See this blog post from Google Chrome’s blog on September 4, 2018.

We launched Google Chrome 10 years ago to give...

As well as this one: As the Chrome dino runs, we caught up with the Googlers who built it

You currently have a special edition of the game out. Can you tell us more about that? Alan: Over the years, the game got a few upgrades, like pterodactyls and night mode. The latest one rolled out this week: a special “anniversary edition” for Chrome’s 10th birthday. Look out for cake, balloons, and a fancy birthday hat.

Downasaur turned 10 this year and the cake is there to celebrate that:

Back in 2008, Google Chrome was revealed to the world via a comic book that detailed the new open source browser. To mark its 10th anniversary this month, Google has updated and themed the whimsical Offline Dino Game.
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The birthday edition allows the dinosaur to collect a birthday cake and wear a party hat, while the background features balloons. This easter egg will be themed for the entire month of September.