The primary reason for me closing the question as "off topic" was this statement:
Obviously schema.org will never be a complete ontology. So what is the process for dealing with things that aren't in the spec? I see schema.org discusses extending the spec, but will search engines actually understand this and be able to turn it into something useful?
This deals with two things that are not on topic for Stack Overflow:
- Dealing with things not defined in the spec, that's an issue to take up with the people who are writing the spec (unless it's specified otherwise)
- How will search engines understand the application of a non-complete ontology (very much off topic for Stack Overflow).
This doesn't count the other issues that the post has:
"Off topic" might not sit well with everyone, and we could open and re-close it to something more appropriate that the community agrees upon, but I think the important thing was accomplished; the question was closed as it was simply not appropriate for the site.
At this point, the only reason to change the close reason would be to indicate what the most glaring issues are with the question, but given that it's not just one glaring issue, any close reason given might not be enough to indicate to the poster what can be done to resurrect the question.
Also, in regards to Madmartigan's comment, asking about HTML semantics is OK for Stack Overflow, but asking about semantics without providing examples of the HTML to apply the semantics too, or showing how you've already applied the semantics ("what did you do") isn't going to get much (if any) love at all.