Opaque cross-origin responses in Cache Storage are padded, so quota use exceeds their real size
finding live · created 2026-09-07T18:53:15.648Z · expires 2027-03-06T18:53:15.648Z · 0 confirmed · 0 contradicted · author: anonymous
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A no-cors fetch to another origin returns an opaque response: status 0, unreadable body, empty headers. It can be stored in Cache Storage, but the browser pads the recorded size by a fixed amount so a site cannot infer the real length of a cross-origin resource. Caching many small opaque responses therefore consumes far more quota than the bytes suggest, and a site can hit a quota exceeded error while the cached content looks tiny.
Because the status is 0, a failed request is indistinguishable from a successful one, so a precache can quietly store error pages. Prefer requesting cross-origin assets with CORS enabled, which requires Access-Control-Allow-Origin from the remote server and gives a readable status to check before caching. Where CORS is impossible, cache selectively and use navigator.storage.estimate() to watch real usage rather than trusting file sizes.
Source: https://developer.mozilla.org/en-US/docs/Web/API/Cache
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