amp-comment-form-yesvalidate.php

<?php
/**
 * AMP Comment Form YesValidate plugin bootstrap.
 *
 * @package   Google\AMP_Comment_Form_YesValidate
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2020 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Comment Form YesValidate
 * Plugin URI: https://gist.github.com/westonruter/a120556c80897d4f7fbf200cb7aa5258
 * Description: Remove novalidate attribute from comment form to force client-side form validation to prevent relying on server-side validation which will not be available during offline commenting. This ensures that background sync wont POST a comment that we already know to be invalid.
 * Version: 0.1
 * Author: Weston Ruter, Google
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/westonruter/a120556c80897d4f7fbf200cb7aa5258
 */

namespace Google\AMP_Comment_Form_YesValidate;

add_filter(
	'amp_content_sanitizers',
	function ( $sanitizers ) {
		require_once __DIR__ . '/Sanitizer.php';
		$sanitizers[ __NAMESPACE__ . '\Sanitizer' ] = [];
		return $sanitizers;
	}
);

Sanitizer.php

<?php
/**
 * Sanitizer file.
 *
 * @package Google\AMP_Comment_Form_YesValidate
 */

namespace Google\AMP_Comment_Form_YesValidate;

use AMP_Base_Sanitizer;
use DOMElement;

/**
 * Class Sanitizer
 *
 * @todo Core should allow for this to be omitted removed.
 */
class Sanitizer extends AMP_Base_Sanitizer {

	/**
	 * Remove the novalidate attribute is removed from the comment form.
	 *
	 * This needs to be made part of the plugin.
	 */
	public function sanitize() {
		/**
		 * Comment form.
		 *
		 * @var DOMElement $form
		 */
		$form = $this->dom->getElementById( 'commentform' );
		if ( $form ) {
			$form->removeAttribute( 'novalidate' );
		}
	}
}

Installation instructions: https://gist.github.com/westonruter/6110fbc4bef0c4b8c021a112012f7e9c