1. A computer-implemented method, comprising: determining, by one or more computers, for each of a plurality of resource requests for a resource that includes first content, whether a supplemental content event has occurred, the occurrence of the supplemental content event indicating that supplemental content will likely be served to a user device after the resource is served to the user device; for each resource request for which the supplemental content event has occurred, serving, by the one or more computers, the resource with a defined supplemental content environment in which the supplemental content is to be displayed, the supplemental content environment being of dimensions that accommodate display of the supplemental content without adjustment of display of the first content upon the user device rendering the resource; and for each resource request for which the supplemental content event has not occurred, serving, by the one or more computers, the resource without the defined supplemental content environment; wherein determining, for each of a plurality of resource requests for a resource that includes first content, whether a supplemental content event has occurred comprises: determining, for the resource, a likelihood that supplemental content will be served; and determining that the supplemental content event has occurred when the likelihood meets a threshold likelihood.