This is hopefully the moment where things have become so absurd (not a criticism of Opera) that it's time for browsers and standards to tackle the problem e.g. look at the proposed -beta flag.
I would prefer an experimental feature not be implemented until at least the syntax has been decided on. Prefixes were implemented so that browsers could have experimental features in place and look where we are now. Adding in a prefix, even a single one, doesn't change the fact that if present then people will use it. If the syntax changes during the -beta prefix phase then lots of stuff will be broken anyway. Can the browser companies at least get their crap together to decide on a basic syntax and then expand on it if needed? Lately they seem to be able to do that with some of the newer properties but we still have prefixes.
Vendor prefixes have turned out to be unsuccessful, but letting browsers move ahead before standards is the best thing that could have happened for the web. And the web apps we see today are evidence of it.
XHTML shows what happens when you try to agree on everything upfront. You mention agreeing on a basic syntax. The browser developers generally do discuss ideas for new features in basic terms before they release them, and hopefully there's some vague sense of agreement between at least two browsers before a feature goes alpha. But agreeing on anything more than that, instead of just hammering out a working implementation, means developers end up with APIs that are hard to use and don't address their users' needs.
Sure, I agree. I just feel that experimental features should be in beta or nightly builds until they are ready for production use. The fact these features were available in current releases means developers will start using them whether they are really ready or not.
Like I said before, they are getting better at agreeing on basic syntax at least. I would say don't make it available until the basics have been decided on. If they are still arguing over details that require a prefix then it isn't ready for release.