My admittedly biased view: I spent two years of my life trying to make the JVM communicate gracefully with Javascript - there were plenty of us at Netscape who thought that bytecode was a better foundation for mobile code. But Sun made it very difficult, building their complete bloated software stack from scratch. They didn't want Java to cooperate with anything else, let alone make it embeddable into another piece of software. They wrote their string handling code in an interpreted language rather than taint themselves with C! As far as I can tell, Sun viewed Netscape - Java's only significant customer at the time - as a mere vector for their Windows replacement fantasies. Anybody who actually tried to use Java would just have to suffer.

Meanwhile Brendan was doing the work of ten engineers and three customer support people, and paying attention to things that mattered to web authors, like mixing JS code into HTML, instant loading, integration with the rest of the browser, and working with other browser vendors to make JS an open standard.

So now JS is the x86 assembler of the web - not as pretty as it might be, but it gets the job done (GWT is the most hilarious case in point). It would be a classic case of worse is better except that Java only looked better from the bottom up. Meanwhile JS turned out to be pretty awesome. Good luck trying to displace it.

SWF was the other interesting bytecode contender, but I don't know much about the history there. Microsoft's x86 virtualization tech was also pretty cool but they couldn't make it stick alone.