1. A hardware system comprising: one or more computer-readable media; software instructions resident on the media which, when executed, are capable of enabling the hardware system to represent a document with a markup representation, wherein the document comprises part of a package that contains multiple payloads, each payload acting as a different representation of the document, the markup representation comprising: a first element that controls how an application reacts to an unknown attribute; a second element that declares that an associated namespace is ignorable; a third element that specifies behavior for ignorable content; a fourth element that reverses the effect of a namespace declared ignorable; a fifth element that specifies content that may be substituted for preferred content if the preferred content is unknown and the preferred content is associated with a namespace requiring that items must be understood; and a sixth element that specifies to document editing tools whether individual attributes in an ignorable namespace should be preserved when the document is modified.
1. A hardware system comprising: one or more computer-readable media; software instructions resident on the media which, when executed, are capable of enabling the hardware system to represent a document with a markup representation, wherein the document comprises part of a package that contains multiple payloads, each payload acting as a different representation of the document, the markup representation comprising: a first element that controls how an application reacts to an unknown attribute; a second element that declares that an associated namespace is ignorable; a third element that specifies behavior for ignorable content; a fourth element that reverses the effect of a namespace declared ignorable; a fifth element that specifies content that may be substituted for preferred content if the preferred content is unknown and the preferred content is associated with a namespace requiring that items must be understood; and a sixth element that specifies to document editing tools whether individual attributes in an ignorable namespace should be preserved when the document is modified. 5. The system of claim 1 , wherein the application is a reader application.