This Part describes a set of XML elements and attributes that collectively enable producers to explicitly guide consumers in their handling of any XML elements and attributes not understood by the consumer.

These elements and attributes enable the creation of future versions of and extensions to this Standard, while enabling desirable compatibility characteristics:

  • A markup producer can produce markup documents that exploit new features defined by versions and extensions, yet remain interoperable with markup consumers thats are unaware of those versions and extensions.

  • For any such markup document, a markup consumer whose implementation is aware of the exploited versions and extensions will deliver functionality that is enhanced by the markup document's use of those versions and extensions.

  • For any such markup document, the markup producer can enable and precisely control graceful degradation that will occur when the markup document is processed by a markup consumer that is unaware of the exploited versions and extensions.