1. A non-transitory computer readable storage device storing instructions that, when executed by one or more data processors, cause the one or more data processors to perform operations comprising: receiving a given search query from a user; providing, in response to receiving the given search query, a search results page that includes a set of search results that link to web pages identified as relevant to the given search query and a set of keyword link units that includes at least a first keyword link unit and a second keyword link unit; receiving a request for a landing page of the first keyword link unit that is initiated by user interaction with the first keyword link unit of the set of keyword link units, wherein the user interaction does not include user interaction with the second keyword link unit; receiving a request for a subset of content items to populate one or more content item slots of the landing page of the first keyword link unit when presented in response to the request for the landing page, wherein the request for the subset of content items to populate the one or more content item slots of the landing page of the first keyword link unit comprises a URL having a first identifier for the first keyword link unit, an assigned first weight, a second identifier for the second keyword link unit, and an assigned second different weight; identifying, in response to receiving the request for the subset of content items, a set of candidate content items for presentation in the one or more content item slots based on the user interaction with the first keyword link and the second keyword link unit that was presented on the search results page but was not interacted with by the user; and in response to receiving the request for the subset of content items to populate one or more content item slots of the landing page of the first keyword link unit, transmitting data to present, in the one or more content item slots of the landing page of the first keyword link unit, at least one of the candidate content items that was identified based on the second keyword link unit that was not interacted with by the user.
1. A non-transitory computer readable storage device storing instructions that, when executed by one or more data processors, cause the one or more data processors to perform operations comprising: receiving a given search query from a user; providing, in response to receiving the given search query, a search results page that includes a set of search results that link to web pages identified as relevant to the given search query and a set of keyword link units that includes at least a first keyword link unit and a second keyword link unit; receiving a request for a landing page of the first keyword link unit that is initiated by user interaction with the first keyword link unit of the set of keyword link units, wherein the user interaction does not include user interaction with the second keyword link unit; receiving a request for a subset of content items to populate one or more content item slots of the landing page of the first keyword link unit when presented in response to the request for the landing page, wherein the request for the subset of content items to populate the one or more content item slots of the landing page of the first keyword link unit comprises a URL having a first identifier for the first keyword link unit, an assigned first weight, a second identifier for the second keyword link unit, and an assigned second different weight; identifying, in response to receiving the request for the subset of content items, a set of candidate content items for presentation in the one or more content item slots based on the user interaction with the first keyword link and the second keyword link unit that was presented on the search results page but was not interacted with by the user; and in response to receiving the request for the subset of content items to populate one or more content item slots of the landing page of the first keyword link unit, transmitting data to present, in the one or more content item slots of the landing page of the first keyword link unit, at least one of the candidate content items that was identified based on the second keyword link unit that was not interacted with by the user. 5. The non-transitory computer readable storage device of claim 1 storing instructions that cause the one or more data processors to further perform operations comprising: transmitting a request for a first-party resource, wherein a first-party resource includes a content item slot to present the set of keyword link units.