Add use case stubs for Cross-Origin Storage (COS) - #1208
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Adds three Stage 1 (use case identification) stubs under guides/performance/ for the WICG Cross-Origin Storage proposal: - avoid-redundant-large-asset-downloads: the imperative navigator.crossOriginStorage.requestFileHandle() API for sharing large files (AI models, Wasm modules, bundled JS libraries, game engine cores) across origins. - load-shared-resources-declaratively: the declarative HTML (crossoriginstorage attribute) and JS (crossOriginStorage import attribute) integrations. - share-web-fonts-across-origins: the declarative CSS cross-origin-storage() request-url-modifier for @font-face. Re-read the explainer (https://github.com/WICG/cross-origin-storage/) and spec (https://wicg.github.io/cross-origin-storage/) in full for this pass; content reflects the current API shape (singular requestFileHandle(), origins upgrade-only semantics, Public Hash List availability gating, GREASE'ing, declarative HTML/JS/CSS integrations). Known blocker: COS has no shipping browser implementation and no entry in the web-features package yet, so the web-feature-ids: cross-origin-storage value in each guide.md is a placeholder that will fail this repo's web-feature-ids CI check. Tracking issue for the upstream feature definition: web-platform-dx/web-features#4029. See PR description for details.
Per feedback: don't call out the web-features dependency in the guide content itself. web-feature-ids still points at the cross-origin-storage placeholder (web-platform-dx/web-features#4029).
Sourced from the working patterns documented in web-ai-community/cross-origin-storage-extension's docs/ folder (progressive-enhancement-demo.html and its CSS/HTML/JS fixtures), rewritten against the literal WICG spec syntax rather than that extension's polyfill-specific shims (e.g. real dynamic import() with a 'with' option instead of the extension's non-standard __non_standard__import() helper, since the guide represents the spec, not one particular polyfill's workaround for it). - avoid-redundant-large-asset-downloads/demo.html: imperative requestFileHandle() read path, NotFoundError/NotAllowedError handling, create:true write path with origins:'*'. - load-shared-resources-declaratively/demo.html: declarative HTML crossoriginstorage attribute on <link>/<script>, and feature-detected dynamic import() with the crossOriginStorage attribute for the JS form. - share-web-fonts-across-origins/demo.html: CSS cross-origin-storage() with a same-list plain url() fallback, COS-enhanced source ordered first per the src list's first-match resolution.
tomayac
marked this pull request as ready for review
August 9, 2026 13:09
rviscomi
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Aug 10, 2026
Co-authored-by: Rick Viscomi <rviscomi@users.noreply.github.com>
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Ah, excellent. Suggestion accepted.
Done (also fixed the built-in AI one). PTAL! Thanks! |
rviscomi
reviewed
Aug 11, 2026
Co-authored-by: Rick Viscomi <rviscomi@users.noreply.github.com>
Co-authored-by: Rick Viscomi <rviscomi@users.noreply.github.com>
rviscomi
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Aug 12, 2026
| name: share-web-fonts-across-origins | ||
| description: Serve large, popular web fonts from a shared cross-origin cache instead of re-downloading them from a font CDN on every site that references them. | ||
| web-feature-ids: | ||
| - cross-origin-storage |
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Suggested change
| - cross-origin-storage | |
| - tmp-cross-origin-storage |
| "web_features": { | ||
| "canvas-html": "ewewraw", | ||
| "container-scroll-state-queries": "bramus", | ||
| "cross-origin-storage": "tomayac", |
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| "cross-origin-storage": "tomayac", | |
| "tmp-cross-origin-storage": "tomayac", |
| name: load-shared-resources-declaratively | ||
| description: Serve popular, unmodified scripts, stylesheets, and JavaScript modules from a shared cross-origin cache using markup or import syntax alone, without writing custom caching logic. | ||
| web-feature-ids: | ||
| - cross-origin-storage |
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| - cross-origin-storage | |
| - tmp-cross-origin-storage |
| name: avoid-redundant-large-asset-downloads | ||
| description: Avoid re-downloading and re-storing large shared assets, such as AI models, Wasm modules, or fully-bundled JavaScript libraries, that a visitor's browser may already hold from an unrelated site. | ||
| web-feature-ids: | ||
| - cross-origin-storage |
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| - cross-origin-storage | |
| - tmp-cross-origin-storage |
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Summary
Adds guidance use cases for Cross-Origin Storage (COS) under
guides/performance/, covering all three ways to use the API:avoid-redundant-large-asset-downloads— the imperativenavigator.crossOriginStorage.requestFileHandle()API: storing/retrieving large files (AI models, Wasm modules, fully-bundled JS libraries, game engine cores) keyed by content hash.load-shared-resources-declaratively— the declarative HTMLcrossoriginstorageattribute (on<link>/<script>elements that already carryintegrity) and thecrossOriginStorageJS import attribute for static/dynamic module imports.share-web-fonts-across-origins— the declarative CSScross-origin-storage()request-url-modifier for@font-face.Each includes a
demo.htmlalongsideguide.mdandexpectations.md.web-feature-idsin all three points atcross-origin-storage(web-platform-dx/web-features#4029).