No, it is not a bug. This is how core handles it. If you look into /wp-includes/comment-template.php, you’ll notice, that the only difference in <form> element, is novalidate attribute added, when current_theme_supports( 'html5', 'comment-form' ) is true. But there are other html elements within comment form, which are affected by theme’s choice of html5 support. For example:
input field for email ( type="email" – html5, type="text" – xhtml ), and input field for url ( type="url" – html5, type="text" – xhtml ).
I would not recommend to remove theme support for html5. WordPress, now, builds our documents with <!DOCTYPE html>, which means, HTML5. If we do remove support, our document will not validate as correct XTML5.
So, how to deal with this offending novalidate attribute? Simple jQuery script will fix it.
Create file removenovalidate.js and put the code below in it:
jQuery( document ).ready(function($) {
$('#commentform').removeAttr('novalidate');
});
Save this file to your theme’s folder. Add the code below to your theme’s functions.php:
function fpw_enqueue_scripts() {
if ( is_single() ) {
wp_register_script( 'fpw_remove_novalidate', get_stylesheet_directory_uri() . '/removenovalidate.js', array( 'jquery' ), false, true );
wp_enqueue_script( 'fpw_remove_novalidate' );
}
}
add_action('wp_enqueue_scripts', 'fpw_enqueue_scripts', 10 );
All done. Your comments form will validate now.