Works like a charm

WordPress 3.5 and .html on PAGES 1.1 worked like a charm for me. I tested in debug mode and didn’t get a single warning—hence the 5 stars.

As a side note: I needed to hack the plugin to output the “.htm” suffix instead of “.html”. Having to do so anyway, I added a (hopefully) unique prefix to all functions and removed the closing PHP tag (?>) at the end of the file for security reasons.

Here’s my final plugin code:

add_action( 'init', 'pageshtm_html_page_permalink', -1 );
register_activation_hook( __FILE__, 'pageshtm_active' );
register_deactivation_hook( __FILE__, 'pageshtm_deactive' );

function pageshtm_html_page_permalink() {
	global $wp_rewrite;
	if ( ! strpos( $wp_rewrite->get_page_permastruct(), '.htm' ) )
		$wp_rewrite->page_structure = $wp_rewrite->page_structure . '.htm';
}
add_filter( 'user_trailingslashit', 'pageshtm_no_page_slash', 66, 2 );

function pageshtm_no_page_slash( $string, $type ) {
	global $wp_rewrite;
	if ( $wp_rewrite->using_permalinks() && $wp_rewrite->use_trailing_slashes==true && $type == 'page' )
		return untrailingslashit( $string );
	else
		return $string;
}

function pageshtm_active() {
	global $wp_rewrite;
	if ( ! strpos( $wp_rewrite->get_page_permastruct(), '.htm' ) )
		$wp_rewrite->page_structure = $wp_rewrite->page_structure . '.htm';

	$wp_rewrite->flush_rules();
}

function pageshtm_deactive() {
	global $wp_rewrite;
	$wp_rewrite->page_structure = str_replace( '.htm', '',$wp_rewrite->page_structure );
	$wp_rewrite->flush_rules();
}