You can add the open attribute into the details tag like this:
<details open>
<summary>Table of Contents</summary>
<ul>
<li><a href="#" class="active">Introduction</a></li>
<li><a href="http://stackoverflow.com/2/">Body</a></li>
<li><a href="http://stackoverflow.com/3/">Conclusion</a></li>
</ul>
</details>The details will then be expanded by default.