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Thomas Steiner @tomayac.com · 29/09/2026

It's early days for the #CrossOriginStorage API, but because it works perfectly as a progressively enhancement, why not teach your agents already how to use it? Modern Web Guidance now has, well, guidance for using the COS API. ⤵️

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Thomas Steiner @tomayac.com · 28/09/2026

You can additionally also comment on the #CrossOriginStorage API feature request bugs. (Please make your comment meaningful, don't just write "me too" or "+1".) 🧭 WebKit: bugs.webkit.org/show_bug.cgi... 🦊 Mozilla: bugzilla.mozilla.org/show_bug.cgi...

bugs.webkit.org

324017 – Support Cross-Origin Storage

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Thomas Steiner @tomayac.com · 28/09/2026

Friendly reminder that there's a way to go on-the-record with your project's or company's #CrossOriginStorage API use case and express support. This can be helpful when discussing with other vendors. 👉 Current supporters: github.com/WICG/cross-o... 👉 Add yours: github.com/WICG/cross-o...

github.com

Issues · WICG/cross-origin-storage

Cross-Origin Storage (COS), a content-addressable browser cache that shares files across origins by hash, with k-anonymity-based availability gating to prevent cross-site tracking - Issues · WICG/c...

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Thomas Steiner @tomayac.com · 21/09/2026

You can now download the macOS binary with this feature (☝️) enabled from github.com/tomayac/Chro.... (Launch Chromium, open `about://flags`, search for Cross-Origin Storage, set it to Enabled, and restart when prompted.) #CrossOriginStorage

github.com

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Thomas Steiner @tomayac.com · 17/09/2026

Another #CrossOriginStorage API DevTools exploration: showing some metadata for resources on the Public Hash List (github.com/WICG/cross-o...), so the entries in the table are more than opaque hash strings.

Chrome DevTools showing multiple resources in Cross-Origin Storage. The metadata of the selected resource (that is on the Public Hash List) shows that the resource is from the list's Hugging Face section and belongs to the Whisper Tiny AI model.

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Thomas Steiner @tomayac.com · 14/09/2026

Akaash Parthasarathy from the WebLLM team has filed WebKit and Mozilla feature request bugs asking for #CrossOriginStorage API support in Firefox and Safari: 🧭 bugs.webkit.org/show_bug.cgi... 🦊 bugzilla.mozilla.org/show_bug.cgi... (Chrome's implementation bug is crbug.com/547587764.)

bugs.webkit.org

324017 – Support Cross-Origin Storage for reuse of WebLLM model artifacts across sites

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Thomas Steiner @tomayac.com · 03/09/2026

🎧 I was a guest of @codepo8.bsky.social again for the @wearedevelopers.bsky.social podcast to talk about the #CrossOriginStorage API: www.wearedevelopers.com/videos/2162-.... Happy listening! (Find all related resources on the Awesome Cross-Origin Storage 😎 list: github.com/tomayac/awes...)

Chris and Tom, both wearing headphone, in a side-by-side view during the podcast recording.

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Reposted by Thomas Steiner

Thomas Steiner @tomayac.com · 01/09/2026

DevTools and browser Settings UI exploration for the #CrossOriginStorage API is coming along nicely. What would you expect to see in both DevTools and Settings? Let me know!

DevTools in Chrome, exploring a resource stored in Cross-Origin Storage.

Chrome Settings page for Cross-Origin Storage, showing the files in Cross-Origin Storage, ordered by size with usage stats for each entry.

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Thomas Steiner @tomayac.com · 01/09/2026

DevTools and browser Settings UI exploration for the #CrossOriginStorage API is coming along nicely. What would you expect to see in both DevTools and Settings? Let me know!

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Thomas Steiner @tomayac.com · 28/08/2026

🤩 Fantastic article by @nucliweb.net on HTTP cache partitioning and the #CrossOriginStorage API: joanleon.dev/en/http-cach.... Try the embedded demos with the COS extension installed or in one of the experimental browser builds (see the Awesome Cross-Origin Storage list: github.com/tomayac/awes...).

joanleon.dev

HTTP cache partitioning: the same image, downloaded twice | Joan León

A page with a cross-site iframe requests the same image from the iframe and from the parent. Chrome downloads it twice; Firefox and Safari, once. Why the cache key differs across the three engines, wh...

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Reposted by Thomas Steiner

Thomas Steiner @tomayac.com · 27/08/2026

I have a new unofficial macOS Chromium build out for you to test with #CrossOriginStorage API support and full DevTools integration: goo.gle/cos-chromium. Remember to set the Cross-Origin-Storage flag in `chrome://flags/#cross-origin-storage` and find Cross-origin storage on the Application tab.

Chromium running Quake with the Chrome DevTools Application panel open and the Cross-origin storage section selected, showing Three.js being shared between two origins.

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Thomas Steiner @tomayac.com · 27/08/2026

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Thomas Steiner @tomayac.com · 26/08/2026

Hah, I know this app, howfuguismybrowser.dev. Worth noting that this app doesn't actually test the functionality of the API, just barely that it exists. You could have the Contact Picker API in `navigator.contacts`, but Chromium then may not necessarily be hooked up to the OS level contacts thingie.

howfuguismybrowser.dev

How Fugu is my browser?

The cross-company effort Project Fugu has the objective of making it possible for Web apps to do anything iOS, Android, or desktop apps can. Find out where on this exciting journey your current browse...

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Thomas Steiner @tomayac.com · 24/08/2026

Early Chrome DevTools exploration for debugging the #CrossOriginStorage API. This shows global stats first and then per-origin stats, here inspecting Three.js. - (WIP) backend CL: chromium-review.googlesource.com/c/chromium/s... - (WIP) frontend CL: chromium-review.googlesource.com/c/devtools/d...

Descent game on the left. The right side shows Chrome DevTools on the Application tab with a new "Cross-origin storage" section.

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Thomas Steiner @tomayac.com · 20/08/2026

macOS users interested in #CrossOriginStorage: you can now run a test build with the API behind a flag 🚩: github.com/tomayac/Chro.... In `about://flags`, search for Cross-Origin Storage, set it to Enabled, restart when prompted. To un-quarantine the app, see github.com/tomayac/Chro....

github.com

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Thomas Steiner @tomayac.com · 18/08/2026

🤩 It's happening!!1! I just sent the "Intent to Prototype" email for the #CrossOriginStorage API: groups.google.com/a/chromium.o.... If you want to stay in the loop, star the ☂️ umbrella Chromium bug crbug.com/547587764 (and the subtask bugs: crbug.com/547587764/de...).

groups.google.com

Intent to Prototype: Cross-Origin Storage

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Thomas Steiner @tomayac.com · 16/08/2026

😎 Chromium (personal fork) passing all Web Platform Tests of the #CrossOriginStorage API. * WIP CL: chromium-review.googlesource.com/c/chromium/s... * Runnable binary: github.com/tomayac/Chro... (Linux x86_64)

Chromium on Linux x86_64 running 30 Web Platform Tests of the Cross-Origin Storage API. All tests pass.

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Thomas Steiner @tomayac.com · 14/08/2026

Look, ma, that's WebKit 🧭 passing all Web Platform Tests for #CrossOriginStorage. And you can try it: github.com/tomayac/WebK... (personal fork)! That's now four prototype implementations of the API in four different browsers: github.com/tomayac/awes... (all of them with releases you can run).

WebKit mini browser showing the passing test suite for the Cross-Origin Storage API: 30 passing tests.

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Thomas Steiner @tomayac.com · 11/08/2026

For the first time you can try the #CrossOriginStorage API on a "real" browser—after the still in-development browsers [shells] Servo and Ladybird: I managed to implement the API in proper Firefox 🦊: github.com/tomayac/awes... (Note that all of those are personal experimental builds.)

github.com

GitHub - tomayac/awesome-cross-origin-storage: A curated list of resources for the Cross-Origin Storage (COS) API

A curated list of resources for the Cross-Origin Storage (COS) API - tomayac/awesome-cross-origin-storage

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Thomas Steiner @tomayac.com · 09/08/2026

See this progressively enhanced demo page for what these proposals would enable: web-ai-community.github.io/cross-origin...! (Try the demo without or with the #CrossOriginStorage extension installed: github.com/web-ai-commu...)

web-ai-community.github.io

Cross-Origin Storage — progressive enhancement demo

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Thomas Steiner @tomayac.com · 09/08/2026

More #CrossOriginStorage API updates: filed HTML issues (CSS already existed) toward better integration in the platform: * Add `crossoriginstorage` attribute to `<link>` and `<script>`: github.com/whatwg/html/... * Add `crossOriginStorage` JavaScript import attribute: github.com/whatwg/html/...

github.com

Add `crossoriginstorage` attribute to `<link>` and `<script>` · Issue #12770 · whatwg/html

What problem are you trying to solve? <link> and <script> elements load resources declaratively, driven entirely by markup, before any page script needs to run. The Cross-Origin Storage (COS) API (...

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Thomas Steiner @tomayac.com · 07/08/2026

I'm keeping track of the learnings in the implementation notes (github.com/WICG/cross-o...), which hopefully will be useful for other vendors. #CrossOriginStorage

github.com

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Thomas Steiner @tomayac.com · 07/08/2026

If you want to experience Cross-Origin Storage (COS) in two real browsers, I managed to add full imperative COS API support (`navigator.crossOriginStorage`) to both @servo.org and Ladybird (personal prototypes): github.com/tomayac/awes.... #CrossOriginStorage

github.com

GitHub - tomayac/awesome-cross-origin-storage: A curated list of resources for the Cross-Origin Storage (COS) API

A curated list of resources for the Cross-Origin Storage (COS) API - tomayac/awesome-cross-origin-storage

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Thomas Steiner @tomayac.com · 06/08/2026

Hope to see some of you at @jsnation.gitnation.org, physically or on streaming! 👋

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Thomas Steiner @tomayac.com · 06/08/2026

Super happy to announce that I'll be presenting about the #CrossOriginStorage API at #JSNationUS 2026: gitnation.com/badges/jsnat.... Click this link to sign up for partial free streaming access! Here's the talk's abstract: gitnation.com/contents/cro....

gitnation.com

Check out my badge & claim your free JSNation US 2026 remote ticket!

Join 5k engineers worldwide at JSNation US 2026 and meet 50+ top speakers at November 16 - 19, 2026

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Thomas Steiner @tomayac.com · 05/08/2026

I've now filed (or commented on existing) standards position requests from other browser vendors (WebKit & Mozilla) and the W3C TAG for the #CrossOriginStorage API: * Mozilla: github.com/mozilla/stan... * WebKit: github.com/WebKit/stand... 🆕 * W3C TAG: github.com/w3ctag/desig... 🆕

github.com

Cross-Origin Storage (COS) · Issue #1427 · mozilla/standards-positions

Specification title Cross-Origin Storage (COS) Specification or proposal URL (if available) No response Explainer URL (if available) https://github.com/WICG/cross-origin-storage/ Proposal author(s)...

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Thomas Steiner @tomayac.com · 05/08/2026

If you're interested in the Cross-Origin Storage API, I just created a new Bluesky feed, check it out and pin it if you like it: bsky.app/profile/did:... #CrossOriginStorage

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Reposted by Thomas Steiner

Web Standards @web-standards.dev · 27/07/2026

Cross-Origin Storage API: stop downloading twice. @morello.dev explains a proposed standard that stores large files by their SHA-256 hash, so origins share one device cache instead of downloading duplicates. #performance #browser morello.dev/blog/cro...

“Cross-Origin Storage” title with the subtitle “Share one copy of a file across origins” above a code editor window and the name Dennis Morello.

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Reposted by Thomas Steiner

Bramus @bram.us · 28/07/2026

Nice to see this being worked on 🤩 You can already use `<input type="checkbox" switch>` today BTW, thanks to this polyfill by my colleague @tomayac.com: github.com/tomayac/inpu...

github.com

GitHub - tomayac/input-switch-polyfill: A polyfill for <input type="checkbox" switch>

A polyfill for <input type="checkbox" switch>. Contribute to tomayac/input-switch-polyfill development by creating an account on GitHub.

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Reposted by Thomas Steiner

Joaquín Sánchez @userquin.bsky.social · 18/07/2026

Integrating Vite PWA Inspector with @vite.dev devtools, the inspector can run also as a seperated middleware (ala vite-plugin-inspect or UnoCSS inspector). @tomayac.com #PWA

Vite PWA Inspector showing PWA web manifest including icons (like chrome devtools) integrated with Vite devtools.

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Thomas Steiner @tomayac.com · 08/07/2026

I just fixed this particular problem, please try again. It should work fine now on both origins.

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Thomas Steiner @tomayac.com · 06/07/2026

Did you read the proposal?

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Thomas Steiner @tomayac.com · 06/07/2026

Just compiling against the latest Flutter should do the job.

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Thomas Steiner @tomayac.com · 04/07/2026

I need to look into the pending PR. Likely I need to make the change there.

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Thomas Steiner @tomayac.com · 03/07/2026

👀 Look, ma! The Cross-Origin Storage extension running on Firefox! If you dare, install it via `about:debugging` from github.com/web-ai-commu.... Else, just wait for it to be approved on the Firefox Add-on Store. More background on #CrossOriginStorage: github.com/WICG/cross-o....

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