[Kukzze]

Closes #46194

Under WHATWG HTML PR whatwg/html#7617, agent clusters are specified to be origin-keyed by default.

This PR updates the documentation:

1. \iles/en-us/web/http/reference/headers/origin-agent-cluster/index.md:
   • Clarifies that modern browsers enable origin-keyed agent clusters by default (?1).
   • Documents \Origin-Agent-Cluster: ?0\ as the explicit opt-out mechanism to relax execution context back to site-keyed isolation.
   • Adds example for \Origin-Agent-Cluster: ?0.
2. \iles/en-us/web/api/document/domain/index.md:
   • Clarifies under failures that setting \document.domain\ is ignored by default in modern origin-keyed environments unless explicitly opted out via \Origin-Agent-Cluster: ?0.

cc @domenic for review as offered in #46194.

[domenic]

Sorry, since filing that issue I have retired, and am no longer available to provide review.

[Kukzze]

No worries at all, thank you for letting me know! Wishing you the very best in your retirement. I'll leave this with the active MDN review rota.

[Josh-Cena]

Words like "Under the current HTML specification" says nothing. MDN always documents the latest spec. What you need to do is add BCD for when that became the case in actual browsers

[Kukzze]

Good point, thanks for catching that! Removed the time-relative spec phrasing across both documents so the behavior is stated directly. I'll also follow up with a PR in browser-compat-data for tracking when browsers enabled origin-keyed agent clusters by default.

[github-actions]

Preview URLs (4 pages)

• /en-US/docs/Web/API/Document/domain
• /en-US/docs/Web/API/Window/originAgentCluster
• /en-US/docs/Web/HTTP/Reference/Headers/Origin-Agent-Cluster
• /en-US/docs/Web/JavaScript/Reference/Execution_model

(comment last updated: 2026-10-31 01:29:11)

[Josh-Cena]

Thank you. The remaining work is in mdn/browser-compat-data#30940