According to the spec, <iframe> should behave like having a Shadow DOM. https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#html-elements-and-their-shadow-dom-subtrees
Created attachment 160680 [details] wip
Created attachment 163407 [details] wip
We don't have a plan to implement this. Marking WONTFIX.