6. A system comprising: one or more processors; one or more memories; computer executable instructions stored on the one or more memories and executable by the one or more processors, the computer executable instructions comprising: a source document having hypertext markup language (HTML) content; an overlay document having extensible hypertext markup language (XHTML) content that specifies changes to be made to an HTML document, wherein the source document specifies an overlay document version with which the source document is compatible, wherein no special tags or other types of markers are required in the source document to enable the changes to be made as specified by the overlay document, wherein when the source document comprises a section of HTML content that specifies that the section is designated as immutable, modification of the section via the overlay document is prevented, the section designated as immutable selected from a group consisting of: attributes of an HTML element of the source document; content of an HTML element of the source document; and structure of an HTML element of the source document; a manifest document that defines a relationship between the source document and the overlay document such that the overlay document is usable for modifying the source document when an attribute condition in the manifest document related to the source document is fulfilled; a document used to upgrade the overlay document from one version to another version; and an interface to utilize the overlay document to modify the presentation of the source document when an associated version of the overlay document is compatible with the source document.
6. A system comprising: one or more processors; one or more memories; computer executable instructions stored on the one or more memories and executable by the one or more processors, the computer executable instructions comprising: a source document having hypertext markup language (HTML) content; an overlay document having extensible hypertext markup language (XHTML) content that specifies changes to be made to an HTML document, wherein the source document specifies an overlay document version with which the source document is compatible, wherein no special tags or other types of markers are required in the source document to enable the changes to be made as specified by the overlay document, wherein when the source document comprises a section of HTML content that specifies that the section is designated as immutable, modification of the section via the overlay document is prevented, the section designated as immutable selected from a group consisting of: attributes of an HTML element of the source document; content of an HTML element of the source document; and structure of an HTML element of the source document; a manifest document that defines a relationship between the source document and the overlay document such that the overlay document is usable for modifying the source document when an attribute condition in the manifest document related to the source document is fulfilled; a document used to upgrade the overlay document from one version to another version; and an interface to utilize the overlay document to modify the presentation of the source document when an associated version of the overlay document is compatible with the source document. 11. The system as recited in claim 6 , wherein the overlay document, when applied to the source document, rearranges elements to modify a structure of the source document.