1. A computer-implemented system implementing a model as a web application, comprising: a computer network, a model database coupled to the computer network, the model database storing: a computational data set defining a relationship between a set of model values; a rendering data set defining the appearance of at least a portion of the set of model values in the web application, the rendering data set including a set of objects, each object being associated with one of the model values of the set of model values and each object of the set including a rendering definition language macro that includes HTML source code that is configured to create at least one user interface element to display its associated model value, each object further including a method for at least one of inputting, outputting and calculating a value of an instance of the model value, and an instance data set including the set of model values of an instance of the web application; a computer coupled to the computer network and to the model database, the computer including: a calculation engine configured to create at least one model value in the instance data set from at least a portion of the set of the model values; and a layout engine configured to create display information from the computational data set, the rendering data set, and the instance data set, the display information including the HTML source code configured to enable the created at least one user interface element and the associated model value to be displayed within at least one web page.