Does ID have to be unique in the whole HTML page?

HTML Web Development Front End Technology

Yes, IDs must be unique in the entire HTML page. Even the official HTML standards suggest the same −

Unique IDs with the id attribute

Example

Let us see an example. Here, we have used the id attribute −

<!DOCTYPE html>
<html>
<head>
   <style>
      #myHeader {
         border: 2px solid yellow;
         background-color: orange;
         padding: 50px;
         text-align: center;
      }
   </style>
</head>
<body>
   <h1 id="myHeader">
      Demo Heading
   </h1>
   <p>This is a text outside.</p>
</body>
</html>

Output

Displaying four different unique IDs

Another example displaying four different unique IDs −

<!DOCTYPE html>
<html>
<head>
   <style>
      #container {
         width: 100%;
         font-size: 10px;
         text-align: center;
      }
      #left {
         float: left;
         width: 100px;
         border: 2px solid green;
      }
      #right {
         float: right;
         width: 100px;
         border: 2px solid orange;
      }
      #center {
         margin: 0 auto;
         width: 100px;
         border: 2px solid red;
      }
   </style>
</head>
<body>
   <div id="container">
      <div id="left">
         <h1>Left</h1>
      </div>
      <div id="right">
         <h1>Right</h1>
      </div>
      <div id="center">
         <h1>Center</h1>
      </div>
   </div>
</body>
</html>