Connection Allowlists (https://chromestatus.com/feature/5175745573945344) is a feature designed to provide explicit control over external endpoints by restricting connections initiated via the Fetch API or other web platform APIs from a document or worker. This aspect of the feature extends Connection Allowlists to support embedded enforcement on iFrames and nested documents/workers. This allows the parent document to enforce restrictions on embedded content where they can choose to opt in or not load. More details on the proposal can be found here: https://github.com/WICG/connection-allowlists
This change allows a path for embedded content with local sources such as srcdoc or data: URLS to have Connection Allowlist support. It also allows a parent document to set/enforce restrictions on nested content such as iframes or workers to ensure that they are at least as strict as the policies they in place. This helps ensure that embedded content cannot escape the restrictions that are in place and bypass the features purpose.
Explainers: https://github.com/WICG/connection-allowlists/blob/main/iframe-embedded-enforcement-explainer.md