Below you will find the styling used by code.gov for its mobile menu
<nav style="background: gray; height: 100px; position: relative">
<div class="mobile-menu-button"><div class="icon icon-menu"></div></div>
</nav>
You can set the default state to open by setting the open attribute to true
<nav style="background: gray; height: 100px; position: relative">
<mobile-menu-button open="true" />
</nav>
<!-- add open to div class when opened -->
<div class="mobile-menu">
<ul>
<li class="search">
<input placeholder="Search Projects..." type="search" aria-label="search">
<button class="go">Go</button>
</li>
<li class="expanded">
<a class="dropdown">About</a>
<ul>
<li>
<a>Source Code Policy</a>
</li>
<li>
<a class="active">Open Source Pilot</a>
</li>
</ul>
</li>
<li>
<a>Privacy Policy</a>
</li>
<li>
<a class="active">Roadmap</a>
</li>
</ul>
</div>
<!-- To active the mobile menu underlay add the active class -->
<div class="mobile-menu-underlay"></div>