An encoding methodology encodes an instance document describing multimedia content by determining a context node in the content description and using a schema associated with the context node to determine the maximum number of children attributes and elements of the context node. Values for required attributes and elements are encoded into required attributes and required elements sections, respectively. Values for each optional attribute and optional element present in the content description are encoded into corresponding optional attributes and optional elements sections. In one aspect, a mask is encoded that indicates which of the optional attributes or optional elements are present in the content description. In another aspect, a count of the optional attributes or optional elements is used in place of the mask. A corresponding decode methodology extracts the values of the attributes and elements from the encoded instance document to re-create the content description.