17. The system of claim 16 , wherein the modification to the output element comprises a modification of a position of the output element.
16. The system of claim 15 , wherein the output presentation comprises a web page; wherein identifying the pattern comprises identifying a layout of the plurality of output elements; wherein determining the modification to the source element corresponding to the modification to the of the plurality of output elements comprises determining a modification to one or more elements of a template file specifying the layout.