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

[mantou132]

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;
}

[Loirooriol]

This could make sense for open shadow trees, but exposing closed ones seems to defeat the closedness?

[mantou132]

Yes, it should only work with open shadow tree