A request for a content document is sent by a client device to a content server through a first network connection. A current network characteristic of the first network connection differs from a configured download constraint to download a content portion of the content document. A root document that omits the content portion of the content document and that includes a content stub is downloaded from the content server. The content stub identifies the content portion using a content identifier and specifies the configured download constraint. A network connection change to the content server from the first network connection to a second network connection that satisfies the configured download constraint to download the content portion of the content document is detected, and the content portion is downloaded using the second network connection and the content identifier within the content stub.