Specified properties MUST be used instead of vendor-defined properties

[akuckartz]

The discussion in OParl#258 (German) illustrates that an expicit statement should be added that specified properties MUST be used instead of vendor-defined properties when the specified properties can express the same information (even when calculations or transformations are required).

Additional requirements for vendor-defined properties might be appropriate.

[akuckartz]

Vendor-specific extensions were discussed regarding CSS. See for example:

Vendor Prefixes Are Hurting the Web
https://hsivonen.fi/vendor-prefixes/

[akuckartz]

http://tools.ietf.org/html/rfc3224 lists reasons to avoid vendor extensions:

   Vendor extensions to standard protocols come at a cost.

      -  Vendor extensions occur without review from the community.
         They may not make good engineering sense in the context of the
         protocol they extend, and the engineers responsible may
         discover this too late.

      -  Vendor extensions preclude interoperation with compliant but
         non-extended implementations.  There is a real danger of
         incompatibility if different implementations support different
         feature sets.

See also:

Considerations on the Extensibility of IETF protocols
http://wiki.tools.ietf.org/html/draft-iesg-vendor-extensions-02