1. A computer system comprising: at least one processor; and memory storing instructions that, when executed by the at least one processor, cause the system to: identify a URL pattern common to a plurality of URLs stored in fetch records; select a sample of URLs from the plurality of URLs; for each URL in the sample, determine whether the URL is optional by: generating a first rendering result for an embedder of the URL using content for the URL, generating a second rendering result for the embedder without using the content for the URL, calculating a similarity score for the URL by comparing the first rendering result to the second rendering result, and determining that the URL is optional when the similarity score is greater than a similarity threshold; and responsive to determining a predetermined quantity of the URLs in the sample are optional, store the URL pattern in a data store of optional resource patterns.