amp-sanitize-processing-instructions.php

<?php
/**
 * AMP Sanitize Processing Instructions
 *
 * @package   AMP_Sanitize_PIs
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP Sanitize Processing Instructions
 * Description: Quick fix for <a href="https://github.com/ampproject/amp-wp/issues/3409">amp-wp#3409</a>.
 * Plugin URI: https://gist.github.com/westonruter/96ae3cb713051aae867b874196eef910
 * Version: 0.1.0
 * 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/96ae3cb713051aae867b874196eef910
 */

namespace AMP_Sanitize_PIs;

add_filter(
	'amp_content_sanitizers',
	function ( $sanitizers ) {
		class PI_Sanitizer extends \AMP_Base_Sanitizer {
			function sanitize() {
				$xpath = new \DOMXPath( $this->dom );
				foreach ( $xpath->query( '//processing-instruction()' ) as $node ) {
					$node->parentNode->removeChild( $node );
				}
			}
		}

		$sanitizers[ __NAMESPACE__ . '\PI_Sanitizer' ] = [];

		return $sanitizers;
	}
);

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