Eventually we will have something like Typed Wikitext or Balanced Templates, which will give us a broader type system for discussing the types of HTML "holes". We "know" we'll have "block" and "inline", but there are also other necessary contexts to allow tables, other types of inline content, etc.

Before we let our extension API out into the world, we should use an extensible naming system for $contexts, instead of a set of booleans.