I just read today that support for the HTML
canvas element has been checked in to the Mozilla source tree. As far as I know, this makes Mozilla the first browser to implement any part of the Web Applications 1.0 specification (still in working draft form at the moment).
For the uninitiated, the WHATWG is a loose collaboration of parties interested in extending current Web behavior to more appropriately support Web applications. This group, which has no relation to the W3C, has published several specifications relating to the extending of HTML. The one that has garnered the most interest to this point is Web Forms 2.0, which calls for extensions to the normal form components for use with Web applications.
Update: Jose correctly pointed out that Safari 2.0 will also support the
canvas element. Looks like Internet Explorer may really be left in the dust sometime soon.