I put up a couple example pages to show the difference; and yes, it is okay to include .html pages in a .php page... I do it all the time.
Example using require_once()
www (dot) michaelkrotchie.com/require_once.php
Code:
<?php
require_once("nothere.html");
?>
Example using include_once()
www (dot) michaelkrotchie.com/include_once.php
Code:
<?php
include_once("nothere.html");
?>
So it displays an error either way, just depends how you want to handle the rest of the page. If you have the file there it doesn't really matter I think.