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!