::slotted()
        
        
          
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      This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.
The ::slotted() CSS pseudo-element represents any element that has been placed into a slot inside an HTML template (see Using templates and slots for more information).
This only works when used inside CSS placed within a shadow DOM. Note that this selector won't select a text node placed into a slot; it only targets actual elements.
Try it
/* 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(template.content.cloneNode(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;
      });
    }
  },
);
/* Selects any element placed inside a slot */
::slotted(*) {
  font-weight: bold;
}
/* Selects any <span> placed inside a slot */
::slotted(span) {
  font-weight: bold;
}
Syntax
::slotted(<compound-selector>) {
  /* ... */
}
Examples
>Highlighting slotted elements
In this example, we use a template with three slots:
<template id="person-template">
  <div>
    <h2>Personal ID Card</h2>
    <slot name="person-name">NAME MISSING</slot>
    <ul>
      <li><slot name="person-age">AGE MISSING</slot></li>
      <li><slot name="person-occupation">OCCUPATION MISSING</slot></li>
    </ul>
  </div>
</template>
We define the <person-details> custom element. In this case, we add styles with JavaScript, though we could have added them in a <style> block within the <template> with the same effect:
customElements.define(
  "person-details",
  class extends HTMLElement {
    constructor() {
      super();
      let template = document.getElementById("person-template");
      let templateContent = template.content;
      const shadowRoot = this.attachShadow({ mode: "open" });
      let style = document.createElement("style");
      style.textContent =
        "div { padding: 10px; border: 1px solid gray; width: 200px; margin: 10px; }" +
        "h2 { margin: 0 0 10px; }" +
        "ul { margin: 0; }" +
        "p { margin: 10px 0; }" +
        "::slotted(*) { color: gray; font-family: sans-serif; } " +
        "::slotted(span) {text-decoration: underline;} ";
      shadowRoot.appendChild(style);
      shadowRoot.appendChild(templateContent.cloneNode(true));
    }
  },
);
When filling the style element with content, you'll see that we select all slotted elements (::slotted(*)) and give them a different font and color. This differentiates them from the slots that haven't been filled. We styled all the slotted <span>s (::slotted(span)) to differentiate the <span>s from the <p>s.
Our markup includes three custom elements, including a custom element with an invalid slot name in a source order that differs from the <template>:
<person-details>
  <p slot="person-name">Wonder Woman</p>
  <span slot="person-age">Immortal</span>
  <span slot="person-occupation">Superhero</span>
</person-details>
<person-details>
  <p slot="person-name">Malala Yousafzai</p>
  <span slot="person-age">17</span>
  <span slot="person-occupation">Activist</span>
</person-details>
<person-details>
  <span slot="person-age">44</span>
  <span slot="not-a-slot-name">Time traveler</span>
  <p slot="person-name">Dr. Who</p>
</person-details>
Result
Specifications
| Specification | 
|---|
| CSS Scoping Module Level 1> # slotted-pseudo>  | 
            
Browser compatibility
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See also
:host:host():host-context():has-slotted- CSS scoping module
 - HTML 
slotattribute - HTML 
<slot>element - HTML 
<template>element - Web components