8. A system comprising: one or more processors; and a memory coupled by the processors comprising instructions executable by the processors, the processors being operable when executing the instructions 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 device; 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 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.
8. A system comprising: one or more processors; and a memory coupled by the processors comprising instructions executable by the processors, the processors being operable when executing the instructions 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 device; 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 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. 14. The system of claim 8 , wherein the processors are further operable when executing the instructions to log, in a data store, one or more of the selected second resources and one or more of the first selected resources that are determined to be required to render the target structured document.