header.css

nav.navigation {
    font-size: 12px;
    position: absolute;
    top: 0; left: 0; right: 0;
    background: #fefefe;
    border-bottom: 1px solid rgba(0, 0, 0, 0.2);
}
nav.navigation ul {
    margin: 0;
    padding: 0;
    display: inline;
}
nav.navigation li {
    display: inline-block;
}
nav.navigation a {
    color: black;
    text-decoration: none;
    padding: 2px 11px;
}
nav.navigation li:hover a.unstyled {
    visibility: hidden;
}
nav.navigation li:hover a.spacer {
    visibility: visible;
}
nav.navigation .active {
    font-weight: bold;
}

nav.navigation .spacer {
    visibility: hidden;
    font-weight: bold;
    position: absolute;
    margin-top: -2px;
}

nav.html

{! Nav links have both an <a> and a hidden <span> to avoid layout shifts
    on hover. !}
{% macro spacedLink(url, text) -%}
<a href="{{url}}" class="spacer">{{text}}</a>
<a href="{{url}}" class="unstyled">{{text}}</a>
{%- endmacro %}

{% macro nav(active) -%}
<nav class="navigation">
  <ul>
    <li class="{{ 'active' if active == 'home' else '' }}">
      {{ spacedLink('/', 'Home') }}
    </li>
    <li class="{{ 'active' if active == 'runs' else '' }}">
      {{ spacedLink('/runs', 'Runs') }}
      {% if active == 'examine' %} &raquo;
      <span class="{{ 'active' if active == 'examine' else '' }}">Examine</span>
      {% endif %}
    </li>
    <li class="{{ 'active' if active == 'callers' else '' }}">
      {{ spacedLink('/callers', 'Callers') }}
    </li>
  </ul>
</nav>
{%- endmacro %}