A computer-implemented method performed by data processing apparatus comprising one or more computers in data communication, the method comprising: receiving, for each resource of a plurality of resources that are each accessible by a user device, action data describing actions supported by each resource, each action being an action that can be performed by the user device processing the resource; for each resource determining, from the action data, an action type for each action supported by the resource, determining, from the action data, action parameters for each action supported by the resource, and indexing the action data, action types and action parameters for the resource in an index that is searchable by a search engine, for each resource, determining whether the user device processing the resource correctly performs the actions described by the action data for the resource, wherein the user device is determined to perform an action correctly when: the resource does not cause an instantiation failure on the user device, and a specified action type for the resource matches an action type observed when the user device processes the resource; for each resource that causes the user device processing the resource to correctly perform the actions described by the action data for the resource, determining that the resource is verified; for each resource that does not cause the user device processing the resource to correctly perform the actions described by the action data for the resource, determining that the resource is not verified; and indexing the determinations of verification for each resource in the index. receiving, for each resource of a plurality of resources that are each accessible by a user device, action data describing actions supported by each resource, each action being an action that can be performed by the user device processing the resource; for each resource determining, from the action data, an action type for each action supported by the resource, determining, from the action data, action parameters for each action supported by the resource, and indexing the action data, action types and action parameters for the resource in an index that is searchable by a search engine, determining, from the action data, an action type for each action supported by the resource, determining, from the action data, action parameters for each action supported by the resource, and indexing the action data, action types and action parameters for the resource in an index that is searchable by a search engine, for each resource, determining whether the user device processing the resource correctly performs the actions described by the action data for the resource, wherein the user device is determined to perform an action correctly when: the resource does not cause an instantiation failure on the user device, and a specified action type for the resource matches an action type observed when the user device processes the resource; the resource does not cause an instantiation failure on the user device, and a specified action type for the resource matches an action type observed when the user device processes the resource; for each resource that causes the user device processing the resource to correctly perform the actions described by the action data for the resource, determining that the resource is verified; for each resource that does not cause the user device processing the resource to correctly perform the actions described by the action data for the resource, determining that the resource is not verified; and indexing the determinations of verification for each resource in the index.