There is an ISO standard that was supposed to address that.  However, since ISO standards are not open access, it's not possible to see what the differences are between that and MARCXML. Open standards always win over closed.
On 3/7/15 7:16 AM, Erik Bertelsen wrote:
In addition to the citation below, please also remind that MARCXML (at least to some people) references MARC21 (formerly USMARC) specifically and does not necessarily support all MARC formats currently in use.
State and University Library Aarhus, DK
Unfortunately, MARCXML is not transitional or transformational. It is a pure serialization of MARC and is not allowed to vary in any way from MARC in ISO 2709 format. MARC and MARCXML are kept in lock step, so none of the advantages of XML over the MARC format are realized.
HAD we moved to XML, leaving 2709 behind, some significant improvements could have been made. Alas...