[NEW ISSUE] 4197 LC Inconsistency on treatment of unknown extensibility elements (Framework, Primer, Guidelines)

See: http://www.w3.org/Bugs/Public/show_bug.cgi?id=4197

Title: Inconsistency on treatment of unknown extensibility elements
Target: Framework, Primer and Guideline documents

Note, the issue and actual proposed changes are included in the .htm 
(with yellow highlighted blocked text) for ease of review.

Thanks.