As Dave notes, this is a “pre-release” in the Open Source, “release early and often” sense. We are not done yet, but the Reference Implementation passes over 90% of the ES3 tests that we host in cvs.mozilla.org. I should note here that the Reference Implementation already handles a great deal of ES4 in addition to most of ES3, so for example the self-hosted built-in classes are mostly there.
We know of some bugs, and we welcome well-written reports of more to fix. If you observe something that might be a bug, but you aren’t ready to file a Trac ticket, feel free to ask for advice on the es4-discuss list.
Extravagant praise to Dave for setting up the site and working on all the details of hosting the source files and binaries. Also to Graydon for tremendous work creating and developing much of the SML code. SML has been a good choice for us, although the performance of our (intentionally not very optimized) reference code can be poky. However, Dave says he is working on a MLton port, which should give a nice performance boost (MLton is an optimizing whole-program SML compiler).
This is just the beginning. Our plan is to finish the Reference Implementation over the summer and then work on specification language to surround pretty-printed excerpts of the SML and self-hosted ES4 code. At the same time, Mozilla, Adobe, and anyone who wants to help will bring up the new language on Tamarin in the new Mercurial repository. I’ll have more to say about that in a bit.