Conflicts with user-defined microdata should be avoidable using multiple (URL-prefixed) names per itemprop, as described in the HTML spec. Multiple itemtypes might also be possible, but the details there are still a bit murky.
And it's the only way you'll be able to convey something like:
<div itemtype="http://www.mediawiki.org/microdata/wikitext/Transclusion" itemscope> <meta itemprop="PageName" content="Template:Foo"> <meta itemprop="RawText" content="Foo#This is some discarded data"> <b>Bar:</b> <span itemprop="argument" itemtype="http://www.mediawiki.org/microdata/wikitext/Argument" itemscope><!-- --><meta itemprop="name" content="bar"><!-- --><meta itemprop="default" content="Baz"><!-- --><span itemprop="value">Foo</span><!-- --></span> </div>
Oh- we do add round-trip / meta-information in data attributes. The use of meta elements to represent otherwise absent or difficult parameters was also discussed earlier in this thread. The idea is to mark up properties inline where it makes sense (and that will be many cases), but revert to meta elements for anything deemed too difficult. I also don't see a need to represent default values as separate itemprops. The fact that some template parameter value came from the default value of an undefined template argument in the page does not seem to be very relevant for its semantics, and can be noted in an attribute as well.
How is the Visual Editor supposed to do that when the dom we're talking about is lossy and doesn't contain any extra metadata giving that information.
We have round-trip information for variable whitespace etc, but that still does not cover changes introduced by the need to transform tag soup into a tree. To minimize the effect of these changes in diffs, we currently plan to only re-serialize parts of the DOM that were actually marked as modified by the editor. Round-trip info contains original source offset ranges for elements, which makes it possible to splice in the original source for untouched DOM parts. The result should be a minimization and localization of any remaining normalization artifacts to avoid 'dirty diffs'- normalization changes in unmodified parts of the document.
Gabriel