Michael Kay wrote
> There are also applications of mixed content that don't fall into the
> classic "marked-up text" model. For example, it can be used to record
> "properties of properties", as in
>
<email><last-modified>2005-02-02</last-modified></emai
l>
To make this fully supported by schemas, one would need a way to
indicate in schema rules for the cardinality, etc., of the "properties
of properties."
But there is also a versioning issue. Since adding such meta-data as the
last-modified date after the fact is a typical usage, attributes today
allow such annotation with less disruption to existing readers. E.g.
<email last-modified='2005-02-02'></email>
But this only works to one level. One cannot add an attribute to that
attribute.
One could imagine a future version of XML that treated elements and
attributes with more parallelism, and allowed attributes to have
structure, perhaps with an element-like syntax, as in:
<email><@last-modified>2005-02-02</@last-modified></em
ail>
And
<email><@last-modified>2005-02-02<@says-who>Joe</@says
-who></@last-modified></email>
