What does “Markup should be rigorous” mean?

Question

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.

Answer 1

rigorous (comparative more rigorous, superlative most rigorous)

1. 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.

Answer 2

Rigorous XML for instance just means valid XML that follows all the markup rules.