chrome/browser/resources/tab_strip/alert_indicators.html

<style>
  :host {
    height: 100%;
  }

  #container {
    display: grid;
    flex-shrink: 0;
    grid-auto-flow: column;
    grid-gap: 4px;
    height: 100%;
  }

  /* Pinned tabs only show the first indicator. */
  :host-context([pinned]) tabstrip-alert-indicator:not(:first-child) {
    display: none;
  }
</style>

<div id="container"></div>