CSSとJavaScriptを使用して垂直タブメニューを作成するにはどうすればよいですか？

CSSとJavaScriptを使用して垂直タブメニューを作成するには、コードは次のとおりです-

例

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
* {
   box-sizing: border-box
}
.tab {
   float: left;
   border: 1px solid blue;
   background-color: black;
   width: 20%;
   height: 200px;
}
.tab button {
   display: block;
   background-color: inherit;
   color: white;
   padding: 22px 16px;
   width: 100%;
   border: none;
   outline: none;
   text-align: left;
   cursor: pointer;
   transition: 0.3s;
   font-size: 17px;
}
.tab button:hover {
   background-color: gray;
}
.tab button.active {
   background-color: #006400;
}
.demo2 {
   float: left;
   padding: 0px 10px;
   border: 1px solid;
   width: 70%;
   border-left: none;
   height: 200px;
}
</style>
</head>
<body>
<h2>Examination</h2>
<p>Following is the exam information:</p>
<div class="tab">
   <button class="demo" onclick="infofunc(event, 'dates')" id="current">Exam Dates</button>
   <button class="demo" onclick="infofunc(event, 'centre')">Exam Centre</button>
</div>
<div id="dates" class="demo2">
   <h3>Dates</h3>
   <p>Exam date will be announced on 25th March.</p>
</div>
<div id="centre" class="demo2">
   <h3>Centre</h3>
   <p>Exam Centre will be announced on 30th March.</p>
</div>
<script>
function infofunc(e, info) {
   var i, content, links;
   content = document.getElementsByClassName("demo2");
   for (i = 0; i < content.length; i++) {
      content[i].style.display = "none";
   }
   links = document.getElementsByClassName("demo");
   for (i = 0; i < links.length; i++) {
      links[i].className = links[i].className.replace(" active", "");
   }
   document.getElementById(info).style.display = "block";
   e.currentTarget.className += " active";
}
document.getElementById("current").click();
</script>
</body>
</html>