CSS Shadow

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

::slotted(.content) {
  background-color: aqua;
}

h2 ::slotted(span) {
  background: silver;
}

<template id="card-template">
  <div>
    <h2><slot name="caption">title goes here</slot></h2>
    <slot name="content">content goes here</slot>
  </div>
</template>

<my-card>
  <span slot="caption">Error</span>
  <p class="content" slot="content">Build failed!</p>
</my-card>

customElements.define(
  "my-card",
  class extends HTMLElement {
    constructor() {
      super();

      const template = document.getElementById("card-template");
      const shadow = this.attachShadow({ mode: "open" });
      shadow.appendChild(document.importNode(template.content, true));

      const elementStyle = document.createElement("style");
      elementStyle.textContent = `
        div {
          width: 200px;
          border: 2px dotted red;
          border-radius: 4px;
        }`;
      shadow.appendChild(elementStyle);

      const cssTab = document.querySelector("#css-output");
      const editorStyle = document.createElement("style");
      editorStyle.textContent = cssTab.textContent;
      shadow.appendChild(editorStyle);
      cssTab.addEventListener("change", () => {
        editorStyle.textContent = cssTab.textContent;
      });
    }
  },
);

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

:host {
  background-color: aqua;
}

<h1 id="shadow-dom-host"></h1>

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;
}

/* 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;
}