Web Forms API

<!DOCTYPE html>
<html>
<body>

<form id="myCarForm">
  Favorite Car: <input type="text" name="fname" value="Volvo">
</form>

<form id="myColorForm">
  Favorite Color: <input type="text" name="favcolor" value="Blue">
</form>

<p>Click the button to display the id of the first form element (index 0) in the document.</p>

<button onclick="myFunction()">Try it</button>

<p id="demo"></p>

<script>
function myFunction() {
  var x = document.forms[0].id;
  document.getElementById("demo").innerHTML = x;
}
</script>

</body>
</html>