Sample contract · MD directive · ARC 2.0.0-beta.28

`::embed`

Embed a supported URL as a sandboxed frame or media block and document refusal fallbacks.

Web Device Markdown directive — not an AUP primitive; not a theme/site layout component.

Sample contract

Field	Value
Purpose	Embed a supported external or same-site URL in content Markdown
Boundaries	Providers limited (YouTube, X/Twitter status, validated same-site); not a general iframe API
Author entry	`::embed{url=…}` or standalone recognized URL
Surface	Web Device document (partial frame path)
Public status	`released`
Fallback / failure	Unrecognized or refused URL → ordinary link / no embed descriptor
Verification	Routes below · `arc --version` = `2.0.0-beta.28`

Source:

::embed{url="https://youtu.be/jNQXAC9IVRw"}

Rendered:

The current renderer turns this supported URL into a responsive, lazy-loaded, sandboxed third-party iframe. Unrecognized URLs stay ordinary links.

Situation	Expected result
Unrecognized host / URL shape	Ordinary link; no embed chrome
Expecting arbitrary iframe with full privileges	Out of contract (`allowSameOrigin` / whitelist rules apply elsewhere)
Same-site path not validated	Refused or link fallback

Item	Value
Baseline	`2026-08-09` · `arc --version` → `2.0.0-beta.28`
Capability row	`docs/aup/capability-matrix.md` §6 · `::embed` · widget `frame`
Implementation	`content-reader.ts` · `content-embed.ts` (youtube / twitter / internal) · `renderer/primitives/frame.ts`
Public routes	`/en/docs/web-device/markdown/embed/` · `/zh/docs/web-device/markdown/embed/`
Local run	`arc service start --blocklet blocklets/arcblock --port 4900` · open routes; refuse path stays link
Accept	Supported URL embeds; unsupported stays link; no general iframe claim