revision:
The value of this attribute must be equal to the id attribute of a <form> element in the same document.
The form attribute can be used on the following elements: <button>, <fieldset>, <input>, <label>, <meter>, <object>, <output>, <select>, <textarea>.
<element form="form_id">
The button below is outside the form element, but still part of the form.
<form style="margin-left:5vw;" action="action_page.php" method="get" id="nameform">
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname"><br><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname">
</form>
<p class="spec">The button below is outside the form element, but still part of the form.</p>
<button style="margin-left:5vw;" type="submit" form="nameform" value="Submit">Submit</button>
The fieldset below is outside the form, but still a part of the form.
<form style="margin-left:5vw;" 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>
<p class="spec">The fieldset below is outside the form, but still a part of the form.</p>
<fieldset style="margin-left:5vw;" form="form1">
<legend>Personalia:</legend>
<label for="fname">First name:</label>
<input type="text" id="fname" name="fname" form="form1"><br>
<label for="lname">Last name:</label>
<input type="text" id="lname" name="lname" form="form1">
</fieldset>