<div class="container">
  <a class="active">ссылка</a>
  <a>ссылка</a>
  <a>ссылка</a>
  <a>ссылка</a>
</div>
.container {
   border-bottom:  2px solid grey;
}
.container a.active {
   border-bottom: 2px solid lightgreen
}

External CSS

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External JavaScript

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