For a while back in the 90s, it seemed like Java would be the last word in interactive web pages, animations, etc etc. Every second web site had some kind of irritating Java applet animated banner.
But it’s faded. In its place we have Flash. It’s not quite write-once, run-anywhere… but it is write once, run on PC, Mac, Linux and Solaris. Which anyway you look at it is just about all of the browser market.
Java still finds a home on mobile devices, and for server development, but is becoming less common on web sites, particularly since it stopped being bundled with IE/Windows.
Perhaps Flash has the upper hand because a lot of web development with heavily interactive content (particularly advertising) is driven by graphic designers and multimedia people rather than programmers, giving Macromedia an advantage over Sun.
Now, if only there could be a ban on developing complete sites in Flash.