What is it? The latest build of Apple's next generation desktop browser, based on their Open Source WebKit framework. Why do we care? Because the very same framework powers the desktop version's baby brother, MobileSafari on the iPhone, and while the 2.0 firmware brought some improvements to the latter, we ain't seen nothing like 4.0 yet.
Quoth Apple Insider:
Safari 4.0's implementation of Cascading Style Sheets (CSS) includes support for masks, gradients, reflections, and specifying a named canvas. Also being carried over from the latest WebKit builds is rudimentary support for the WAI-ARIA (Rich Internet Application) and cross-site XMLHttpRequest specifications. Among the technologies supported from HTML 5 are the ability to send messages between documents, storage of data either locally or just for the user's session, the option of running web applications outside of a browser or when disconnected from the network, and canvas pixel manipulation.
This is some serious tech, and personally we can't wait for it to filter down into our multi-touching little hands!