I'm not sure how you're categorizing "modern browsers", but this is disappointing for any developers who hope to reach audiences on Windows RT or Windows Phone 8 without needing to make an app for those platforms.
As mentioned below in another comment reply I made, that was a poor usage of "modern browsers". I meant like Chrome, Safari, and Firefox. Also, I agree that it's disappointing, but I also don't think it's necessarily a "deal breaker" for the features that IE10 is missing. However, that will depend on the style of HTML5 game that you are building.
