Every man is enthusiastic at times.
One man has enthusiasm for thirty minutes, another man has it for thirty days.
But it is the man who has it for thirty years who makes a success in life. (Edward B. Butler)
Apple's own web site says "build your own widgets using the JavaScript language", but that's sort of misleading. The widgets are HTML+CSS+JS. They are not some JS-only thing.
In other words, each widget is just a web page, and so you have the full power of WebKit behind each one... CSS2, DOM2, JS, HTML, XMLHttpRequest, Flash, Quicktime, Java, etc.