1. A computer-implemented method of learning characteristics of product variables within web pages of a merchant, the method comprising: receiving a product feed identifying a product offered by the merchant and having a value for a product variable associated with the product; obtaining, from the merchant, a web page associated with the product; producing a model representing the web page, the model having a set of nodes representing portions of data of the web page; identifying, by a processor, candidate nodes of the model that match the product feed value for the product variable; determining, from the candidate nodes, product variable characteristics expected of a node representing the product variable; storing the determined product variable characteristics; and extracting a value for the product variable from a new web page of the merchant, at least in part using the learned product variable characteristics.