<form> ... </form>
<form method=post > ... </form>
<form method=post >
Name: <input name=firstName>
<input type=submit>
</form>
<form method=post action="execute.php">
Name: <input name=firstName>
<button>Submit</button>
</form>
<?php
echo <<< _START
<form method=post action="thisFile.php">
Name: <input name=firstName>
<button>Submit</button>
</form>
_START;
echo "<pre>";
print_r($_POST);
echo "</pre>";
?>
There is no need or reason to spend time styling this file. Keep styling simple, if you have it at all, and inside a SINGLE set of <style> tags
This form should not include development notes that user can see and should be beautifully styled (ie, NO PRE TAGS). You may place css rules inside a single pair of <style> tags in the same file OR place all of it in a linked css file. Do not use ANY inline styling; just tags with classes (and possibly id's though these should be very very few. No more than 2.)
IDEALLY, this will be only two files, one file for each of 1 & 2 above. However, if you've linked in your css there may be more. Do not separate form from php processing. All in one!