Jeremy Keith, on his personal blog:
It is entirely possible—nay, desirable—to use features long before they are supported in every browser. That’s how we move the web forward. If we waited until there was universal support for a feature before we used it, we’d still be using CSS 1.0 and HTML 2.0.
We agree. For our broad features we do our best to make sure our services work in the widest array of browsers possible. However, we are not afraid, nor will ever hold back from, using the most cutting edge features of the web simply because some percentage of people using old terminals at their desk job can’t use them.
Move the web forward.