Technology changes, the future can't be predicted, and often specification writers will have to publish something that they expect to change in the future, simply because there is an urgent need for a standard. Hopefully they will have gotten the extensibility right…
But in general, stability of a specification is a desirable characteristic. Having to re-learn how to do something is costly, creating new programs to do the same thing in a different way is costly, and converting existing documents and other resources to a different format is also costly, so changes with little or no benefit should be avoided.
The Web is a revolution. But let it not be one that eats its children. For all its shortcomings, HTML is quite a useful little format. There is absolutely no need to replace it yet.