WordPress Template Part in iFrame

create a .php file within the root WP folder:

<?php
  define('WP_USE_THEMES', false);
  require('wp-blog-header.php');
  wp_nav_menu(array('theme_location' => 'my_menu')); // change it with your menu location
?>

and call it in your .html page (as a iframe). These only gets you the menu HTML, so you’ll also need to import the theme CSS styles. Not sure how iframe content is styled, you might need to add them inside the .php file too…

There are other solutions, like using javascript to request your homepage HTML, then get the menu element from the results and append it into your html-powered website