It should be a unique ID, No other element has the same ID on the same page. You can use the ID attribute to point CSS and JavaScript.
syntax:
<tagname id="unique_name"></tagname>
Example:
<!DOCTYPE html>
<html>
<head>
<title>Class</title>
<style>
#imp_id{
font-size: 22px;
}
</style>
</head>
<body>
<div id="imp_id">Hello World!</div>
</body>
</html>
- Above Example <div id="imp_id">Hello World!</div>
- The id name is case sensitive!