serviceworker for iframes with srcdoc

As I understand it <iframe seamless="seamless" sandbox="allow-same-origin" srcdoc="<img src='/example.png'/>"/> should load /example.png as if it was on the containing page.

To this end, I feel like the navigation should go via the parent page's service worker's fetch event.

Unfortunately this doesn't work in either Chrome or Firefox because the srcDoc has no document.URI, you can see the current behaviour here: https://cdn.rawgit.com/ConradIrwin/2194f982494ba943a4c0/raw/ec0a6d3ebe69e2953e10d049005065de85588fa0/iframe.html

I'm trying to understand if this is a deliberate feature "iframes with srcdoc should not inherit the parent service-worker under any circumstance", or an edge-case that wasn't considered.

My use-case is loading emails into an iframe, I'd like to be able to fetch resources in the emails using the service worker so that I can load attachments as inline images.

Start with the linked iframe.html reproduction and inspect how the srcdoc iframe requests /example.png in Chrome and Firefox. Review the service worker fetch behavior described in the issue, then determine whether the specification should define inheritance from the containing page. Done means reaching a clear standards decision, not just reproducing the behavior.