1. In an extensible markup language document stored on a computer-readable memory medium, wherein said document comprises plural elements including a media object element used in developing and executing a multimedia presentation by a computer, an action element tag which is readable by the computer from the memory medium and which defines an action, comprising: an object parameter indicating a media object element identified elsewhere in said document; an attribute parameter indicating an attribute of the indicated media object element; and a value parameter indicating a value of the attribute, wherein the action element tag represents a function to change a value of the attribute indicated by the attribute parameter of the media object element indicated by the object parameter to the value indicated by the value parameter.