ese-website/_includes/header.njk

<header>
  <div class="header-container content">
    <a id="header-logo" href="{{ "/" | locale_url }}" title="home">
      <img alt="ESE {{year}} Logo" src="/static/ESELogo24.svg">
    </a>
    <nav id="mainNav">
      {{ collections.mainNav | pageLang | eleventyNavigation | eleventyNavigationToHtml | safe }}
    </nav>
  </div>
</header>

{% css %}
  {% include "header.css" %}
{% endcss %}