Methods, systems, and apparatus, including computer programs encoded on a computer-readable storage medium, and including a method for predicting visibility of web pages. The method comprises identifying web pages that include slots for presenting content. The method further comprises, for each slot, creating a fingerprint based on a URL associated with the page and structural information associated with the slot's location on the page, determining a likelihood of the slot being visible when the web page is initially presented, the likelihood in the form of a predictive model, and saving the fingerprint and predictive model in a data structure. The method further comprises receiving a request for content from a web page, creating fingerprints associated with the web page, and comparing the fingerprints to fingerprints in the data structure. When a matching fingerprint is located, prediction data is used that is associated with a matching fingerprint for the web page.