The moment it's out there (that is, not behind a flag), you cannot fix it, update it etc. because people will be relying on the observed behaviour.
The hard part is advertising the feature (at the very least to developers) because there are just too many featurs in modern browsers. Edit: for example, Chrome has Origin Trials https://developer.chrome.com/origintrials/#/trials/active for experimental features.
But you do it quickly, within a short time of launching it. Anyone who was able to make use of it within a couple months, will be able to adapt and fix things.
Once it's been there for a year, then sure, people rely on it. But there's no reason to take so long.
No, you can't. Not simply.
> But you do it quickly, within a short time of launching it. Anyone who was able to make use of it within a couple months, will be able to adapt and fix things.
That's not how the web works. "People who make use of it in a few months" could easily apps and sites serving millions of people. This is a process that applies not only to image codecs, but to, say hardware APIs and interfaces, network protocols etc.
> But there's no reason to take so long.
There are very many reasons from technical to political to anything in between (and sideways, too).
Or, if it's not behind a flag, then you can tell people not to use it, but they will anyway, which is exactly what you want, because then you get feedback. And when you change it, nothing really bad happens.
Sounds like the flag method is a failure, and just releasing it actually works.
In JPEG XL case it failed not because it was behind a flag, and the failure of one feature doesn't mean that the entire system isn't working.
> Sounds like the flag method is a failure, and just releasing it actually works.
No, "just releasing" doesn't work for the reasons I outlined above.