:defined

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<simple-custom text="Custom element example text"></simple-custom>

<p>Standard paragraph example text</p>

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/* Give the `p` elements distinctive background */
p {
  background: yellow;
}

/* Both the custom and the built-in element are given italic text */
:defined {
  font-style: italic;
}

/* Only simple-custom element is applied with green background*/
simple-custom:defined {
  display: block;
  background: green;
}