[Jellby]

It looks like a nested list is meant:

<ul>
<li>Colophon</li>
<li>Capitoli
    <ul id="spoiler">
    <li>Capitolo 1</li>
    <li>Capitolo 2</li>
    ...
    </ul></li>
<li>Archivio Audio</li>
</ul>

Or you should apply the display and onclick properties to the individual items:

<ul>
<li>Colophon</li>
<li>Capitoli</li>
<li class="spoiler" style="display:none">Capitolo 1</li>
<li class="spoiler" style="display:none">Capitolo 2</li>
...
<li>Archivio Audio</li>
</ul>

(and modify the javascript so that it alters the display property for each and every li element with class="spoiler")