Cloning an element with shadow DOM can produce broken clones when elements in the shadow are implemented with derived C++ types. That is because these elements are cloned based on their tag names, not their actual C++ types. This is a tracking bug for fixing elements that fall into this trap.
We need to go through the dependent bugs, including closed ones, and:
1. Where there's a landed patch that overrides cloneElementWithoutAttributesAndChildren, determine if that is needed since the rollout in r88137. If not, we should remove the override. Don't revert the whole patch, though, since they contain useful tests.
Sometimes the landed regression tests are just for crash/element doesn't appear type regressions. They don't verify the functionality of cloned elements. We need to add verify that cloned elements are functional too.
2. Where there's no landed patch on a bug we should write a regression test that covers cloning and the functionality of cloned elements. These may expose functionality bugs, which we should file and fix. Close the bug when the test is landed and happy on the bots.