Updates to a content description represented as a tree are contained in a series of fragment update units. Each fragment update unit includes an update command and a navigation path, and may include a fragment payload depending on the update command. The update command is applied to a set of nodes in the tree that are selected using the navigation path. The fragment payload may contain a fragment, a fragment reference, or an attribute. The update command may be an add, delete or replace command. More than one fragment payload may be present in the fragment update unit, with each fragment payload applied to a different one of the nodes from the set selected by the navigation path.