The ISO definition of generalized markup states:
Markup should be rigorous so that the techniques available for processing rigorously-defined objects like programs and databases can be used for processing documents as well.
What does "rigorous" mean in this context?
I found a paper which says:
...the type definition and the marked up document together [...] constitute the rigorously described document that machine processing requires.
...but I'm still unclear on the exact definition.
rigorous (comparative more rigorous, superlative most rigorous)
- Manifesting, exercising, or favoring rigour; allowing no abatement or mitigation; scrupulously accurate; exact; strict; severe; relentless; as, a rigorous officer of justice; a rigorous execution of law; a rigorous definition or demonstration.
I.e., the rules for a markup language should be specified in such a way as to leave no ambiguity and no doubt as to their interpretation.