:host-context()

/* Following CSS is being applied inside the shadow DOM. */

:host-context(.container) {
  border: 5px dashed green;
}

:host-context(h1) {
  color: red;
}

<!-- elements outside shadow dom -->
<div class="container">
  <h1 id="shadow-dom-host"></h1>
</div>

const shadowDom = init();

// add a <span> element in the shadow DOM
const span = document.createElement("span");
span.textContent = "Inside shadow DOM";
shadowDom.appendChild(span);

// attach shadow DOM to the #shadow-dom-host element
function init() {
  const host = document.getElementById("shadow-dom-host");
  const shadowDom = host.attachShadow({ mode: "open" });

  const cssTab = document.querySelector("#css-output");
  const shadowStyle = document.createElement("style");
  shadowStyle.textContent = cssTab.textContent;
  shadowDom.appendChild(shadowStyle);

  cssTab.addEventListener("change", () => {
    shadowStyle.textContent = cssTab.textContent;
  });
  return shadowDom;
}

/* 指定されたセレクター引数の子孫である場合にのみ、
   シャドウルートホストを選択します。 */
:host-context(h1) {
  font-weight: bold;
}

/* .dark-theme クラスが文書本体に適用されている場合に、
   段落のテキストの色を黒から白に変更します。 */
p {
  color: #000;
}

:host-context(body.dark-theme) p {
  color: #fff;
}

:host-context(<compound-selector>) {
  /* ... */
}

<h1>
  ホストセレクターの<a href="#"><context-span>例</context-span></a>
</h1>

const style = document.createElement("style");
const span = document.createElement("span");
span.textContent = this.textContent;

const shadowRoot = this.attachShadow({ mode: "open" });
shadowRoot.appendChild(style);
shadowRoot.appendChild(span);

style.textContent =
  "span:hover { text-decoration: underline; }" +
  ":host-context(h1) { font-style: italic; }" +
  ':host-context(h1):after { content: " - no links in headers!" }' +
  ":host(.footer) { color : red; }" +
  ":host { background: rgb(0 0 0 / 10%); padding: 2px 5px; }";