Good tips everybody, thanks! A brief summary:

SERVER-SIDE PROCESSING (GET and POST)
  <!-- If your form submission WILL NOT WRITE anything on the SERVER-SIDE, use GET -->
  <form action="action.php" method="get">

  <!-- If your form submission WILL WRITE some data on the SERVER-SIDE, use POST -->
  <form action="action.php" method="post">

CLIENT-SIDE PROCESSING (ID and NAME)
  <!-- If the CLIENT-SIDE NEEDS to access or modify this field, use the id attribute -->
  <input id="firstName" name="firstName" type="text">

    <!-- create a script that runs on the CLIENT-SIDE to access and modify the field -->
    <script type="javascript">
      var firstName = document.getElementById("firstName");
      firstName.value = 'some text';
      firstName.focus();
    </script>

  <!-- If the CLIENT-SIDE DOES NOT NEED to access or modify this field, -->
  <!-- then there is no need for the ID attribute or a client side script -->
  <input name="firstName" type="text">

Note the HTML SYNTAX:
  - In <form>, 'get' and 'post' are the VALUE of the attribute named 'method'.
  - In <input>, 'id' and 'name' are the NAME of the attribute with values of 'firstName'.
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