1. A computer-implemented method, comprising: receiving, at a primary video content server including one or more processors and memory, via a network, a video document, the video document including video content to be provided for presentation and embedded with an encoding specifying one or more content slot locations; determining, by the primary video content server, based on the encoding embedded in the video content, the one or more content slot locations at which to insert content slots, each of the content slots configured to receive content items for presentation, each of the content slot locations having a start time corresponding to a time of the video document at which to insert the content slot; inserting, by the primary video content server, the content slots at the determined content slot locations of the video document, the content slots configured to receive content items; receiving, at the primary video content server, from a client device, a request to provide the video document for presentation; and delivering, by the primary video content server, responsive to receiving the request to provide the video document, to the client device, the video document for presentation on the client device, receipt of the video document causing the client device to: (a) initiate playing the video document using a content rendering application, (b) execute, subsequent to initiating playing of the video document, the encoding embedded in the video document specifying the one or more content slot locations, (c) generate, based on executing the encoding embedded in the video document, a request for supplemental video content to insert at the start time corresponding to the time of the video document at which to insert the content slot, and (d) transmit, to a secondary video content server including one or more processors and memory, the request for supplemental video content to insert in the content slot, receipt of the request for supplemental video content causing the secondary video content server to: (i) select a content item from the plurality of content items to serve in the content slot of the video document based on the request for supplemental video content and (ii) provide, to the client device, responsive to selecting the video content item, the video content item for presentation on the client device at the time of the video document at which the content slot is inserted; and (e) insert, responsive to receiving the video content item from the secondary video content server, for playing at the client device, the video content item into the content slot specified by the encoding embedded in the video content document.
1. A computer-implemented method, comprising: receiving, at a primary video content server including one or more processors and memory, via a network, a video document, the video document including video content to be provided for presentation and embedded with an encoding specifying one or more content slot locations; determining, by the primary video content server, based on the encoding embedded in the video content, the one or more content slot locations at which to insert content slots, each of the content slots configured to receive content items for presentation, each of the content slot locations having a start time corresponding to a time of the video document at which to insert the content slot; inserting, by the primary video content server, the content slots at the determined content slot locations of the video document, the content slots configured to receive content items; receiving, at the primary video content server, from a client device, a request to provide the video document for presentation; and delivering, by the primary video content server, responsive to receiving the request to provide the video document, to the client device, the video document for presentation on the client device, receipt of the video document causing the client device to: (a) initiate playing the video document using a content rendering application, (b) execute, subsequent to initiating playing of the video document, the encoding embedded in the video document specifying the one or more content slot locations, (c) generate, based on executing the encoding embedded in the video document, a request for supplemental video content to insert at the start time corresponding to the time of the video document at which to insert the content slot, and (d) transmit, to a secondary video content server including one or more processors and memory, the request for supplemental video content to insert in the content slot, receipt of the request for supplemental video content causing the secondary video content server to: (i) select a content item from the plurality of content items to serve in the content slot of the video document based on the request for supplemental video content and (ii) provide, to the client device, responsive to selecting the video content item, the video content item for presentation on the client device at the time of the video document at which the content slot is inserted; and (e) insert, responsive to receiving the video content item from the secondary video content server, for playing at the client device, the video content item into the content slot specified by the encoding embedded in the video content document. 7. The computer-implemented method of claim 1 , wherein determining the one or more content slot locations at which to insert content slots includes analyzing content associated with the video document and identifying suitable content item locations based on the analyzed content.