Thanks so much for your response, Eric. Good thought! — here's the Login page sans the site's design template — and it appears fine.
users_LogIn_clean.php
Here's another sample interior page where (like all others), the form and template render perfectly:
users_Registration.php
I'll keep eyeballing the HTML, but if there is anything that jumps out at you, kindly let me know. Thanks again for your time and assistance. Greatly appreciated!