[...]

Both are custom solutions for internal use. For an external interface, a
standardized solution would be preferable. HTML5 microdata [2] seems to
fit our needs quite well.

Assuming a template that expands to a div and some content, this would
be represented like this:

<div itemscope
    itemtype='http://en.wikipedia.org/wiki/Template:Sometemplate' >
    <h2>A static header from the template</h2>
    <!-- The template argument 'name', expanded in the template -->
    <p itemprop='name' content='The wikitext name'>The rendered name</p>
</div>

In this case, an expanded template argument within (for example) an
infobox is identified inside the template-provided HTML structure, which
could enable in-place editing.

Unused arguments (which are not found in the template expansion) or
unexpanded templates can be represented using non-displaying meta elements:

<div itemscope
    itemtype='http://en.wikipedia.org/wiki/Template:Sometemplate'
    id='uid-1' >
    <h2>A static header from the template</h2>
    <!-- The template argument 'name', expanded in the template -->
    <p itemprop='name' content='The wikitext name'>The rendered name</p>
    <meta itemprop='firstname' content='The wikitext firstname'>
</div>

The itemref mechanism can be used to tie together template data from a
single template that does not expand to a single subtree:

<div itemscope itemref='uid-1'>
  <!-- Some more template output from expansion of
http://en.wikipedia.org/wiki/Template:Sometemplate -->
</div>