<form action="/action_page.php" method="get" id="form1">
  <label for="favcolor">What is your favorite color?</label>
  <input type="text" id="favcolor" name="favcolor">
  <input type="submit">
</form>

<fieldset form="form1">
  <legend>Personalia:</legend>
  <label for="fname">First name:</label>
  <input type="text" id="fname" name="fname" form="form1"><br><br>
  <label for="lname">Last name:</label>
  <input type="text" id="lname" name="lname" form="form1">
</fieldset>