How to force a list to be vertical using html css

I would add this to the LI's CSS

.list-item
{
    float: left;
    clear: left;
}

Try putting display: block in the <li> tags instead of the <ul>

Hope this is your structure:

   <ul> 
     <li>
        <div ><img.. /><p>text</p></div>
    </li>
     <li>
        <div ><img.. /><p>text</p></div>
    </li>
     <li>
        <div ><img.. /><p>text</p></div>
    </li>
   </ul> 

By default, it will be add one after another row:

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if you want to make it vertical, just add float left to li, give width and height, make sure that content will not break the width:

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li
{
   display:block;
   float:left;
   width:300px; /* adjust */
   height:150px; /* adjust */
   padding: 5px; /*adjust*/
}