However, with a clear view of where extensibility occurs, extension may break any content formatted to older creole versions.

For example, if a new creole uses the tilde as escape character, or assigns special meaning to leading hyphens (as discussed in the [[List Markup Alternatives]]), then older content will break that contains tilde or leading hyphens without any markup intentions.

I therefore believe that at least for the case of text-based markup, we need an extensibility container, compare [[Generic Extension Element Proposal]].

Gregor Hagedorn - 2007-03-09