It is an attribute of all html elements, used to uniquely identify Html elements. it gives unique identity to html elements. Duplicate Id should not be used with html elements which means
same id must not be used with other html elements. In stylesheet or in head section, id is prefixed
with # symbol then name without space.
Example
Example:
<!DOCTYPE html>
<html>
<head>
<style>
#d1 {
background-color: red;
color: black;
width: 100px;
height: 50px;
padding:20px;
}
#s1 {
background-color: green;
color: black;
width: 100px;
height: 50px;
padding:20px;
}
</style>
</head>
<body>
<div id="d1">
id example with red background
</div>
<span id="s2">
id example with green background
</span>
</body>
</html>