Remember the Cross-Origin-Embedder-Policy (COEP) and the Cross-Origin-Opener-Policy (COOP) headers for making your site cross-origin isolated? If not, here's my colleague Eiji Kitamura's article Making your website "cross-origin isolated" using COOP and COEP . To be effective, they need to be sent as in the example below.

cross-origin-embedder-policy: credentialless
cross-origin-opener-policy: same-origin

Cross-origin isolated documents operate with fewer restrictions when using the following APIs:

SharedArrayBuffer can be created and sent via a Window.postMessage() or a MessagePort.postMessage() call. Performance.now() offers better precision. Performance.measureUserAgentSpecificMemory() can be called.

Typically, sending non-default HTTP headers like COOP and COEP means controlling the server so you can configure it to send them. I recently learned that they are also honored if set through a service worker 🤯! This means you can make apps on static hosting like on GitHub Pages cross-origin isolated!

• Blog post by stefnotch: Enabling COOP/COEP without touching the server
• Library by Guido Zuidhof: coi-serviceworker
• Demo: Are we cross-origin isolated?

One example where cross-origin isolating your site is needed is with SQLite Wasm when you want to use persistent storage with the origin private file system virtual file system called OPFS sqlite3_vfs. I'm glad to have this coi-serviceworker trick up my sleeve now, and you do, too!