8. A system, comprising: a data processing apparatus; and a computer readable memory device storing instructions that cause the data processing apparatus to perform operations comprising: receiving, in response to a selection of a search result referencing a first webpage resource, content of the first webpage resource, wherein the first webpage resource is determined to be responsive to a search query included in a search request, the search query being one or more terms that are submitted to a search engine as input to a search operation, and the search result having been provided in response to the search request; receiving a candidate set of search query suggestions for the search query, each search query suggestion in the candidate set being one or more terms that may be submitted to a search engine as input to a search operation, the candidate set of search query suggestions based, in part, on search history data that includes data from and related to previous search requests associated with the search query; for each of the search query suggestions in the candidate set of search query suggestions that meet a relevancy criterion that defines the search query suggestion as being relevant to the content of the first resource, the relevancy criterion being a relevance score that measures a relevance of the search query suggestion to the content of the first webpage resource displayed on a user device, including the search query suggestion in a final set of search query suggestions so that the final set of search query suggestions include only search query suggestions that meet the relevancy criterion; and providing the final set of search query suggestions for display on the user device.
14. The system of claim 8 , further comprising: receiving content of a subsequent resource for which the first resource is an ancestor in a link graph representing the subsequent resource and first resource as nodes; determining that the content of the subsequent resource meets an evaluation relevance threshold and in response to the determination, for each of the search query suggestions in the candidate set of search query suggestions: determining whether the search query suggestion meets a relevancy criterion that defines the search query suggestion as being relevant to the content of the subsequent resource; for each search query suggestion that meets the relevancy criterion, including the search query suggestion in a subsequent final set of search query suggestions; and providing the subsequent final set of search query suggestions for display on the user device.