Missing isn't broken: data validation and freedom on the Semantic Web "Missing isn't broken. In the general case, you are free to say as much, or as little, in your RDF document as you like. RDF vocabularies such as FOAF, Dublin Core, MusicBrainz, RDF-Wordnet don't get to tell you what to do, what to write, what to say. Instead, they serve as an interconnected dictionary documenting the meaning of the terms you're using in your RDF documents."
"If the stark notion of 'valid -vs- invalid' document checking doesn't make sense in the decentralised Semantic Web environment, how can we make things easier for developers who are trying to work with this free-flowing mix of RDF markup? If nothing is mandatory, then how can they write code that knows what to expect?.
There are several answers here. The first is that, if we want this to scale to the planet, we have to accept that one size won't fit all, that different parties will want to say quite varying things in their FOAF documents, and that our ability to impose our views on their documents is limited."