It's a black or white thing. If you choose not to follow the standard, then you are not conformant. Unfortunately, either because of omissions in the spec or confusion on the part of developers, there has apparently been some difference of opinion as to how some nodes should behave. In the past this has been used as an excuse for poor conformance. Hopefully, these ambiguities are being addressed in the current amendment to the spec and conformance issues should become much clearer.
Interoperability is a different thing.
Once conformance issues have been addressed, interoperability becomes a question of how to implement things that are outside of the spec. The question of how this will be done will be one of the key questions affecting the future of VRML as a workable multi-browser format. Perhaps a way will be found to make profiles work that will allow any browser to display any content with the addition of a small bit of extra code. All rights reserved.