chromium/chrome/browser/resources/privacy_sandbox/internals/cr_frame_list.html

<style>
  :host {
    /*display: flex;
    flex-direction: column;
    width: fit-content;*/
    --tabs-background-color: #fbfbfb;
    --tabs-border-color: #c8c8c8;
    --tabs-hover-text-color: black;
    --tabs-unselected-text-color: #646464;
  }

  #tablist {
    background: var(--tabs-background-color);
    border-right: 1px solid var(--tabs-border-color);
    border-bottom: 1px solid var(--tabs-border-color);
    display: inline-block;
    margin: 0;
    padding-inline-start: 9px;
    padding-top: 14px;
    vertical-align: top;
  }

  :host([sticky-tabs]) #tablist {
    position: sticky;
    top: 0;
    z-index: var(--cr-tab-box-tabs-z-index, 1);
  }

  #tablist ::slotted(*) {
    background: var(--tabs-background-color);
    border-bottom: 0;
    border-radius: 0;
    cursor: default;
    display: block;
    margin-inline-start: 0;
    padding: 4px 9px 4px 10px;
    transition: none;
  }

  #tablist ::slotted(:not([selected])) {
    color: var(--tabs-unselected-text-color);
  }

  #tablist ::slotted(:not([selected]):hover) {
    color: var(--tabs-hover-text-color);
  }

  #tablist ::slotted([selected]) {
    font-weight: bold;
    margin-bottom: -1px;
    position: relative;
    transition: none;
    z-index: 0;
  }

  #tablist:focus {
    outline: none;
  }

  #tabpanels {
    box-shadow: none;
    display: inline-block;
    flex: 1;
    overflow: hidden;
    vertical-align: top;
  }

  #tabpanels ::slotted(*) {
    display: none;
    flex: 1;
  }

  #tabpanels ::slotted([selected]) {
    display: block;
  }
</style>

<span id="tablist" role="tablist">
  <slot name="tab"></slot>
</span>
<span id="tabpanels">
  <slot name="panel"></slot>
</span>