CodePen

HTML

            
              <ul>
  <li>1</li>
  <li>2</li>
  <li>3</li>
  <li>4</li>
  <li>5</li>
</ul>
            
          
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CSS

            
              ul:after {
  content: '';
  width: 0;
  height: 100%;
  position: absolute;
  border: 1px solid black;
  top: 0;
  left: 100px;
}
            
          
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JS

            
              
            
          
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999px
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