Extensible By Omission

When specifying markup, we can specify what markup will have a particular effect, and what markup will not have any effect. If we say nothing it all, that markup may have an effect in future releases. The specification is "extensible by omission" because we haven't specified everything. Some things have been left undefined to be defined later.

Example: In HTML, em and strong elements can be nested. If we just specify that **foo** will result in bold text, and //bar// will result in italic text, and we don't specify whether these will nest or not, then we are extensible by omission. Obviously they don't have to nest right now, but they might have to nest in a later revision of the specification.