Document extensible Markdown iframe code block editors

[ntrogh]

Product change

• Source: Add extensible Markdown iframe editors vscode#333992
• Merged: 2026-09-02T18:22:52Z

Markdown extensions can contribute experimental iframe-based code block editors with external HTML entry points, optional host communication, reusable iframe runtimes, and workspace trust restrictions. The manifest includes an optional runtimeKey from 1 to 256 characters.

Why the docs need an update

api/extension-guides/markdown-extension.md covers Markdown preview styles, plugins, and scripts but does not describe this experimental contribution point, its manifest, host transport, runtime reuse, or trust behavior.

Suggested documentation

• api/extension-guides/markdown-extension.md: Add an experimental code block editor section that documents the contribution manifest, external HTML entry point, runtimeKey constraints, host communication, runtime reuse, and workspace trust behavior.
• api/extension-guides/overview.md: Add navigation if needed for discoverability.

Details to verify

• The feature is experimental.
• runtimeKey is optional and must contain from 1 to 256 characters when provided.
• Contributed editors are subject to workspace trust restrictions.

Acceptance criteria

• Extension authors can configure an experimental Markdown code block editor from the guide.
• Manifest fields, constraints, communication behavior, runtime reuse, and trust restrictions match the implementation.