r88077 [1] added a default XML namespace (xmlns attribute on the root node) for the API's XML output.
In r99135 [2] this was partially backed out to avoid clobbering some modules that specified their own XML namespaces in output, such as RSD discovery info. With this change a query parameter needed to be added to include the namespace, but it would still clobber if you used it on affected modules.
In r101912 [3] I took the parameter out, returning almost to r88077's behavior -- but swapping the order of things so an xmlns contained in the output data will take precedence, avoiding the clobbering.
However I do want to confirm that this is an OK thing to do; as noted in a CR comment on [1] this may actually break XPath queries assuming the non-namespaced data. If there is a compat issue, we need to make sure it's documented and communicated...
[1] https://www.mediawiki.org/wiki/Special:Code/MediaWiki/88007 [2] https://www.mediawiki.org/wiki/Special:Code/MediaWiki/99135 [3] https://www.mediawiki.org/wiki/Special:Code/MediaWiki/101912
-- brion