Comment ajouter une bordure CSS

ul {
  display: block;
  width: 255px;
  list-style: none;
  margin: 0;
  padding: 0;
  font-size: 0;
  line-height: 0;
}

ul li {
  font-size: 16px;
  box-sizing: border-box;
  line-height: 2;
  padding: 0 10px;
  width: 100%;
  white-space: nowrap;
  text-overflow: ellipsis;
  overflow: hidden;
  display: inline-block;
  border: 1px solid #007ab6;
  border-bottom-color: transparent;
  border-top-color: transparent;
  background: #ebf7fd;
  position: relative;
  z-index: 1;
  margin: -1px 0;
}

ul li:first-child {border-top-color: #007ab6;}
ul li:last-child {border-bottom-color: #007ab6;}

ul li:hover {
  overflow: visible;
  width: auto;
  min-width: 100%;
  border-color: #007ab6;
  margin: 0;
  z-index: 0;
  line-height: calc(2em - 2px);
}

ul li:first-child:hover, 
ul li:last-child:hover {
  margin: -1px 0; 
  line-height: 2; 
}

<ul>
  <li><a href="">someloooooooooooooooooooooong text </a></li>
  <li><a href="">text </a></li>
  <li><a href="">someloooooooooooooooooooooong text </a></li>
  <li><a href="">someloooooooooooooooooooooong text </a></li>
</ul>