In Bug 77503, an element becomes able to have multiple shadow root. Next step is to ensure they are rendered correctly. We should confirm: (1) The youngest shadow root is rendered correctly. (2) <content> element is rendered correctly. Currently we don't have <shadow> element. Supporting <shadow> element will be another step.
The description is not clear. We have fuzzers. We've written many tests. Let's reopen when we get some clearer idea about this.