CodePen

HTML

            
              <div class="nav">
    <ul id="links">
        <li id="item_1">Item1</li>
        <li id="item_2">Item2</li>
        <li id="item_3">Item3</li>
        <li id="item_4">Item4</li>
    </ul>       
</div>

<div id="newpop">
    <div id="pop_item_1">
        <span class="class_1">TEST 1</span>
        <p class="new_1">1111111111</p>
        <div class="new_2">2222222</div>
        <p class="new_1">33333333333</p>
        <div class="new_2">444444444</div>
    </div>  
   <div id="pop_item_2">
        <span class="class_1">TEST 2</span>
        <p class="new_1">1111111111</p>
        <div class="new_2">2222222</div>
        <p class="new_1">33333333333</p>
        <div class="new_2">444444444</div>
    </div>  
    <div id="pop_item_3">
        <span class="class_1">TEST 3</span>
        <p class="new_1">1111111111</p>
        <div class="new_2">2222222</div>
        <p class="new_1">33333333333</p>
        <div class="new_2">444444444</div>
    </div>  
    <div id="pop_item_4">
        <span class="class_1">TEST 4</span>
        <p class="new_1">1111111111</p>
        <div class="new_2">2222222</div>
        <p class="new_1">33333333333</p>
        <div class="new_2">444444444</div>
    </div>  
</div>

            
          
!

↑ Insert the most common viewport meta tag

CSS

            
              #newpop div { display:none; }
#newpop div.show { display:block; }
ul#links li { cursor:pointer;}

.nav{
    display:block;
    float:left;
    border:solid 1px #000;
    padding:1em;
    overflow:hidden;
}
#newpop{
    overflow:hidden;
    padding-left:.25em;
}
#newpop div{
    border:solid 1px #000;
    padding:1em;
}

            
          
!
? ?
? ?
Must be a valid URL.
+ add another resource
via CSS Lint

JS

            
              var _ul = document.getElementById('links');

for (var i = 0; i < _ul.children.length; i++) {
    _ul.children[i].onclick = function(ev) {
        s = document.getElementById('pop_' + this.id);
        newpop = document.getElementsByClassName('show');
        var shown = s.className == 'show';
        
        for (var j = 0; j < newpop.length; j++) {
            newpop[j].className = '';
        }
        s.className = shown ? '' : 'show';
    };
}
            
          
!
Must be a valid URL.
+ add another resource
via JS Hint
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