Thanks for sharing your approach. In the end, I decided the API results didn't need to be persisted in storage -- they get stored in a local variable and jetti

Thanks for sharing your approach. In the end, I decided the API results didn't need to be persisted in storage -- they get stored in a local variable and jettisoned with Javascript's garbage collection. My primary concern was: to-what scope did the data belong? (Check my other comments to see my dissatisfaction with Chrome's dev-doc's language on this topic.) I decided that the user history did not need to be precisely-known, so - my strategy here was to hash the url, cut the hash to a much shorter string (to increase likelihood of collisions), and then pad the shortened-hash with a random number of characters on either side, and concatenate that string to a large blob of text. The extension would then be able to perform an indexOf search on that blob to be reasonably sure that the user had been to that URL. This uses localStorage. (This keeps the extension from repeatedly querying commonly-visited URLs but also does not prove you were at any given URL since collisions are expected to happen.)