<div>
  <my-element>
    <p>
      Slotted content
    </p>
  </my-element>
</div>

p {
  margin: 0;
}

import {
  LitElement,
  html,
  css
} from "https://cdn.skypack.dev/lit-element@2.4.0";

window.customElements.define(
  "my-element",
  class extends LitElement {
    static get styles() {
      return css`
        div {
          border: 1px solid red;
        }
      `;
    }
    render() {
      return html`
        <div>
          <p>Inner content</p>
        </div>
        <div>
          <slot></slot>
        </div>
      `;
    }
  }
);