13. One or more computer storage media encoded with computer-executable instructions that, when executed, direct a computing system to perform acts comprising: identifying an overlay document to be applied to a source document, wherein the source document specifies one or more overlay versions with which the source file is compatible, wherein when an associated version of the overlay document is not compatible with the source document, an associated document is utilized to upgrade the overlay document to another version, wherein no special tags or other types of markers are required in the source document to enable application of the overlay document, wherein when the source document comprises a section of 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 element of the source document; content of an element of the source document; and structure of an element of the source document; generating a landmark hierarchy based on landmark definitions included in the overlay document, where each landmark of the landmark hierarchy represents one of either a deterministic or a relative reference to a location of one or more elements that are present in the source document, wherein the one or more elements each have attributes corresponding to a definition of each landmark in the overlay document; stepping through elements in the source document to identify an element in the source document that corresponds to a first landmark in the landmark hierarchy; identifying an overlay directive associated with the first landmark, an overlay directive being chosen from a plurality of directives; and generating a modified element by applying the identified overlay directive to the element in the source document that corresponds to the first landmark in the landmark hierarchy.
13. One or more computer storage media encoded with computer-executable instructions that, when executed, direct a computing system to perform acts comprising: identifying an overlay document to be applied to a source document, wherein the source document specifies one or more overlay versions with which the source file is compatible, wherein when an associated version of the overlay document is not compatible with the source document, an associated document is utilized to upgrade the overlay document to another version, wherein no special tags or other types of markers are required in the source document to enable application of the overlay document, wherein when the source document comprises a section of 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 element of the source document; content of an element of the source document; and structure of an element of the source document; generating a landmark hierarchy based on landmark definitions included in the overlay document, where each landmark of the landmark hierarchy represents one of either a deterministic or a relative reference to a location of one or more elements that are present in the source document, wherein the one or more elements each have attributes corresponding to a definition of each landmark in the overlay document; stepping through elements in the source document to identify an element in the source document that corresponds to a first landmark in the landmark hierarchy; identifying an overlay directive associated with the first landmark, an overlay directive being chosen from a plurality of directives; and generating a modified element by applying the identified overlay directive to the element in the source document that corresponds to the first landmark in the landmark hierarchy. 16. The one or more computer storage media as recited in claim 13 , wherein the overlay directive comprises instructions to modify an attribute of the element in the source document that corresponds to the first landmark in the landmark hierarchy.