Below is an interesting use of the id attribute. It is used within the tag and used to identify the form for elements outside of the boundaries so that they will be included with the other fields within the form.
<form action="action_page.php" id="form1">
First name: <input type="text" name="fname"><br>
<input type="submit" value="Submit">
</form>
<p>The "Last name" field below is outside the form element, but still part of the form.</p>
Last name: <input type="text" name="lname" form="form1">