15. One or more computer-readable non-transitory storage media embodying software operable when executed to: receive a request from a client device for a target structured document; in response to the received request from the client device and in a first response phase: access a data structure comprising an entry for the target structured document and one or more first resources associated with the target structured document; for each of one or more of the first resources associated with the target structured document: compute a probability for the first resource that represents a likelihood that the first resource will be included in a response to a future request for the target structured document; compare the probability to a first predetermined threshold; and when the probability exceeds the first predetermined threshold, identify the first resource as a selected first resource for the target structured document; generate a first response portion comprising one or more of: one or more of the selected first resources; or one or more references to one or more of the selected first resources; send the first response portion to the client; and further in response to the received request from the client device and in a second response phase taking place after sending the first response portion: for each of one or more of the second resources associated with the target structured document: compute a probability for the second resource that represents a likelihood that the second resource will be included in a response to a future request for the target structured document; compare the probability to a second predetermined threshold; and when the probability exceeds the second predetermined threshold, identify the second resource as a selected second resource for the target structured document; generate a second response portion comprising structured document language code for the target structured document and one or more of: one or more of the selected second resources associated with the target structured document; or one or more references to one or more of the selected second resources; and send the second response portion to the same client device.
15. One or more computer-readable non-transitory storage media embodying software operable when executed to: receive a request from a client device for a target structured document; in response to the received request from the client device and in a first response phase: access a data structure comprising an entry for the target structured document and one or more first resources associated with the target structured document; for each of one or more of the first resources associated with the target structured document: compute a probability for the first resource that represents a likelihood that the first resource will be included in a response to a future request for the target structured document; compare the probability to a first predetermined threshold; and when the probability exceeds the first predetermined threshold, identify the first resource as a selected first resource for the target structured document; generate a first response portion comprising one or more of: one or more of the selected first resources; or one or more references to one or more of the selected first resources; send the first response portion to the client; and further in response to the received request from the client device and in a second response phase taking place after sending the first response portion: for each of one or more of the second resources associated with the target structured document: compute a probability for the second resource that represents a likelihood that the second resource will be included in a response to a future request for the target structured document; compare the probability to a second predetermined threshold; and when the probability exceeds the second predetermined threshold, identify the second resource as a selected second resource for the target structured document; generate a second response portion comprising structured document language code for the target structured document and one or more of: one or more of the selected second resources associated with the target structured document; or one or more references to one or more of the selected second resources; and send the second response portion to the same client device. 17. The media of claim 15 , wherein none of the selected second resources are identical to any of the selected first resources.