:defined

customElements.define(
  "simple-custom",
  class extends HTMLElement {
    constructor() {
      super();

      let divElem = document.createElement("div");
      divElem.textContent = this.getAttribute("text");

      let shadowRoot = this.attachShadow({ mode: "open" }).appendChild(divElem);
    }
  },
);

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

<p>Standard paragraph example text</p>

// 为两个元素设置不同的背景色
p {
  background: yellow;
}

simple-custom {
  background: cyan;
}

// 将自定义元素和内置元素的字体都设为斜体
:defined {
  font-style: italic;
}

simple-custom:not(:defined) {
  display: none;
}

simple-custom:defined {
  display: block;
}