A request for content is received at a content server via a first network connection from a client device. The content includes a set of portions represented within a root document that are each configurable with assigned download constraints that specify network characteristics under which the individual portions of the content are downloadable. A determination to defer at least one portion of the content for download is made based upon a network characteristic of the first network connection not satisfying a configured download constraint assigned to the at least one portion of the content. The root document is sent with each deferred portion of the content replaced with a content stub within the root document. Each content stub identifies the respective deferred portion of the content as a deferred content portion and includes a deferred content portion identifier of the respective deferred portion of the content.