shadow-part

// src/components/shadow-part/index.hbs

<link rel="stylesheet" href="src/components/shadow-part/index.css">

<template id="tabbed-custom-element">
  <style type="text/css">
    *,
    ::before,
    ::after {
      box-sizing: border-box;
      padding: 1rem;
    }

    :host {
      display: flex;
    }
  </style>
  <div part="tab active">Tab 1</div>
  <div part="tab">Tab 2</div>
  <div part="tab">Tab 3</div>
</template>

<tabbed-custom-element></tabbed-custom-element>