<h1 class="p-5 alert alert-dark text-center">
  <img src="https://upload.wikimedia.org/wikipedia/commons/thumb/8/80/Wikipedia-logo-v2.svg/225px-Wikipedia-logo-v2.svg.png" style="width:100px" />
  <p>Search Article on Wikipedia via  Wikipedia  Search API</p>
</h1>  
<div class="mt-2 p-5">
            <div class="row pb-4">
              <div class="col-md-10">
                <input type="text" id="sTerm" class="form-control" placeholder="Type here to search an article on wikipedia... By Pawan Mall | www.pawanmall.net">
              </div>
              <div class="col-md-2">
                <button type="button" id="searchArticle" class="btn btn-primary btn-block">Search</button>
              </div>
            </div>
            <div id="resultArea" class="form-control border-0" contenteditable="false"></div>
          </div>

You need to include bootstrap 4 css file...
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">

You need for jQuery file to perform the search task...
<script src="https://code.jquery.com/jquery-3.5.0.min.js" integrity="sha256-xNzN2a4ltkB44Mc/Jz3pT4iU1cmeR0FkXs4pru/JxaQ=" crossorigin="anonymous"></script>