How do I center align horizontal <UL> menu?

Question

From http://pmob.co.uk/pob/centred-float.htm:

The premise is simple and basically just involves a widthless float wrapper that is floated to the left and then shifted off screen to the left width position:relative; left:-50%. Next the nested inner element is reversed and a relative position of +50% is applied. This has the effect of placing the element dead in the center. Relative positioning maintains the flow and allows other content to flow underneath.

Code

#buttons{
    float:right;
    position:relative;
    left:-50%;
    text-align:left;
}
#buttons ul{
    list-style:none;
    position:relative;
    left:50%;
}

#buttons li{float:left;position:relative;}/* ie needs position:relative here*/

#buttons a{
    text-decoration:none;
    margin:10px;
    background:red;
    float:left;
    border:2px outset blue;
    color:#fff;
    padding:2px 5px;
    text-align:center;
    white-space:nowrap;
}
#buttons a:hover{ border:2px inset blue;color:red;background:#f2f2f2;}
#content{overflow:hidden}/* hide horizontal scrollbar*/

<div id="buttons">
    <ul>
        <li><a href="#">Button 1</a></li>
        <li><a href="#">Button 2's a bit longer</a></li>
        <li><a href="#">Butt 3</a></li>
        <li><a href="#">Button 4</a></li>
    </ul>
</div>

This works for me. If I haven't misconstrued your question, you might give it a try.

    div#centerDiv {
        width: 100%;
        text-align: center;
        border: 1px solid red;
    }
    ul.centerUL {
        margin: 2px auto;
        line-height: 1.4;
        padding-left: 0;
    }
    .centerUL li {
        display: inline;
        text-align: center;
    }

<div id="centerDiv">
    <ul class="centerUL">
        <li><a href="http://www.amazon.com">Amazon 1</a>&nbsp;&nbsp;</li>
        <li><a href="http://www.amazon.com">Amazon 2</a>&nbsp;&nbsp;</li>
        <li><a href="http://www.amazon.com">Amazon 3</a></li>
    </ul>
</div>

With CSS3 flexbox. Simple.

ul {
  display: flex;
  justify-content: center;
}

ul li {
  padding: 0 8px;
}

This is the simplest way I found. I used your html. The padding is just to reset browser defaults.

ul {
  text-align: center;
  padding: 0;
}
li {
  display: inline-block;
}

<div class="topmenu-design">
  <!-- Top menu content: START -->
  <ul id="topmenu firstlevel">
    <li class="firstli" id="node_id_64">
      <div><a href="#"><span>Om kampanjen</span></a>
      </div>
    </li>
    <li id="node_id_65">
      <div><a href="#"><span>Fakta om inneklima</span></a>
      </div>
    </li>
    <li class="lastli" id="node_id_66">
      <div><a href="#"><span>Statistikk</span></a>
      </div>
    </li>
  </ul>
  <!-- Top menu content: END -->
</div>

Here's a good article on how to do it in a pretty rock-solid way, without any hacks and full cross-browser support. Works for me:

--> http://matthewjamestaylor.com/blog/beautiful-css-centered-menus-no-hacks-full-cross-browser-support

Try this:

div.topmenu-design ul
{
  display:block;
  width:600px; /* or whatever width value */
  margin:0px auto;
}