Regarding section 1.5 itself: The Open Office TC decided to use the term
MAY rather than MUST (or will) at the mentioned location, because it
wanted to ensure that the OpenDocument specification can be used by as
many implementations as possible. This means that the format should also
be usable by applications that only support a very small subset of the
specification, as long as the information that these applications store
can be represented using the OpenDocument format. A requirement that all
foreign elements and attributes must be preserved actually would mean
that some applications may not use the format, although the format
itself would be suitable. Therefor, we leave it up to the
implementations, which elements and attributes of the specification they
support, and whether they preserve foreign element and attributes. Some
more information about this can be found in appendix D of the specification.