Thank you all for your comments, I accepted Pete's and Amit's answer, as it helped me a great deal. If anyone needs to accomplish the same as I do, here's some adaptations of what I'm going for:
This one is practically the answer given here:
Html:
<ul class="menutree">
<li>
<div class="ou" style="background-color:#B2D1FF;">
<div class="ou_content">
Title<br/>
Other data etc<br/>
<a href="#">Link</a>
</div>
<input type="checkbox" id="item-0-0-1" /><label for="item-0-0-1"> V </label>
<br/>
<ul>
<li>
<div class="ou" style="background-color:#99C2FF;">
<div class="ou_content">
Title<br/>
Other data<br/>
<a href="#">Link</a>
</div>
</div>
</li>
<li>
<div class="ou" style="background-color:#99C2FF;">
<div class="ou_content">
Title<br/>
Other data<br/>
<a href="#">Link</a>
</div>
</div>
</li>
</ul>
</div>
</li>
</ul>
And here is css for above:
(note I had to add the br to the selector)
body {
text-align: center;
}
ul {
display: inline-block;
padding:0px;
margin:0px;
text-align:center;
}
li {
display: inline-block;
list-style: none;
vertical-align:top;
margin-top:20px;
margin-right:10px;
text-align:center;
}
div.ou {
border:1px solid blue;
margin:0px;
margin-right:10px;
padding-top:0px;
padding-left:20px;
padding-bottom:20px;
padding-right:0px;
border-radius:10px;
text-align:left;
min-width:100px;
}
div.ou_content {
min-width:150px;
padding:4px;
border:0px solid green;
margin-top:10px;
margin-right:10px;
}
label {
border: 1px solid navy;
float:right;
margin-right:0px;
margin-left:10px;
display:inline-block;
cursor:pointer;
background-color:lightblue;
}
#menutree li {
list-style: none;
}
.css-treeview input + label + br + ul
{
display: none;
}
.css-treeview input:checked + label + br + ul
{
display: inline-block;
text-align: center;
}
.css-treeview input
{
opacity: 0;
display:none;
}
Also I did another proof-of-concept:
Here's my own try-out:
Html:
<ol id="menutree">
<li>
<div class="contentdiv"> place for content </div>
<label for="c3" class="menu_label">Title1 V</label>
<input type="checkbox" id="c3" name="level2" value="1" />
<ol>
<li>
<div class="contentdiv"> place for content </div>
<label for="c4" class="menu_label">Title2</label>
<input type="checkbox" id="c4" name="level3" value="1" />
</li>
<li>
<div class="contentdiv"> place for content </div>
<label for="c5" class="menu_label">Title3</label>
<input type="checkbox" id="c5" name="level3" value="1" />
</li>
</ol>
</li>
</ol>
And css for this:
#menutree li {
list-style: none;
text-align: center;
}
li .menu_label + input[type=checkbox] {
opacity: 0;
display:none;
}
li .menu_label {
cursor: pointer;
}
li .menu_label + input[type=checkbox] + ol > li {
display: none;
}
li .menu_label + input[type=checkbox]:checked + ol > li {
display: inline-block;
text-align: center;
}
li :not(.menu_label + input[type=checkbox]:checked + ol > li) {
display: none;
}
div.contentdiv {
height:40px;
padding-top:6px;
}
li {
border-top:4px solid blue;
border-right: 2px solid blue;
border-left: 2px solid blue;
border-bottom: 0px solid blue;
border-radius:30px 30px 0px 0px;
margin-top:20px;
margin-left:6px;
margin-right:6px;
padding-right:10px;
padding-left:10px;
display: inline-block;
vertical-align:top;
text-align: center;
height:50px;
min-width:100px;
}
label {
border: 1px solid navy;
padding:0px;
border-radius:0px;
vertical-align:top;
text-align: center;
margin-left:4px;
margin-right:4px;
}
body {
text-align: center;
}
ol {
display: block;
padding:0px;
margin:0px;
}