1. srcdoc attribute for iframes

    Override the content specified in the `src` attribute (if present) with HTML content within the attribute.

  2. element api: setattribute: requires `value` parameter to be `trustedhtml` instance (for `iframe.srcdoc`), or `trustedscripturl` (for html `script.src`, and svg `script.href`) when trusted types are enforced

  3. htmliframeelement api: srcdoc

  4. htmliframeelement api: srcdoc: requires `trustedhtml` instance when trusted types are enforced

  5. html element: iframe: srcdoc