[selectors] Consider adding a selector to match the element that has shadow DOM

Add the following code to remind users that the undefined elements are used:

:not(:defined) {
  display: block;
  font-size: 0;
}
:not(:defined) * {
  display: none;
}
:not(:defined)::before {
  font-size: 1rem;
  display: block;
  content: 'The element is not defined';
  padding: 1em;
  text-align: center;
  background: gray;
}

But now we have Declarative Shadow DOM, we can no longer determine which elements may have layout problems(demo), to find these elements, it is suggested to add a pseudo class::attached:

:not(:defined, :attached) {
  display: block;
  font-size: 0;
}
:not(:defined, :attached) * {
  display: none;
}
:not(:defined, :attached)::before {
  font-size: 1rem;
  display: block;
  content: 'The element is not defined';
  padding: 1em;
  text-align: center;
  background: gray;
}

Review the proposed :attached pseudo-class alongside the linked Declarative Shadow DOM discussion in WHATWG DOM issue 831 and the provided demo. Determine the selector’s intended behavior for elements with shadow DOM, then document the specification changes and validation needed; no repository files or tests are named in the issue.