Prototype for https://github.com/ampproject/amp-wp/issues/4528

amp-on-tap-attribute-fix.php

<?php
/**
 * AMP On Tap Attribute Fix plugin initialization file.
 *
 * @package   AMP_On_Tap_Attribute_Fix
 * @author    Weston Ruter, Google
 * @link      https://gist.github.com/westonruter/19f18bf520d0b8c37e39d17b82febd2f
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: AMP On Tap Attribute Fix
 * Plugin URI:  https://gist.github.com/westonruter/19f18bf520d0b8c37e39d17b82febd2f
 * Description: Automatically inject the required <code>role=button</code> and <code>tabindex=0</code> attributes for non-tabbable elements with <code>on="tap:..."</code>. This will be made part of the AMP plugin in <a href="https://github.com/ampproject/amp-wp/issues/4528">amp-wp#4528</a>.
 * 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
 */

namespace AMP_On_Tap_Attribute_Fix;

/**
 * Filter sanitizers.
 *
 * @param array $sanitizers Sanitizers.
 * @return array Sanitizers.
 */
function filter_sanitizers( $sanitizers ) {
	if ( version_compare( AMP__VERSION, '1.5', '<' ) ) {
		return $sanitizers;
	}

	require_once __DIR__ . '/class-sanitizer.php';
	$sanitizers[ __NAMESPACE__ . '\Sanitizer' ] = [];
	return $sanitizers;
}
add_filter( 'amp_content_sanitizers', __NAMESPACE__ . '\filter_sanitizers' );

class-sanitizer.php

<?php
/**
 * Sanitizer
 *
 * @package AMP_On_Tap_Attribute_Fix
 */

namespace AMP_On_Tap_Attribute_Fix;

use AMP_Base_Sanitizer;
use DOMElement;

/**
 * Class Undo_JS_Lazy_Loaded_Images_Sanitizer
 */
class Sanitizer extends AMP_Base_Sanitizer {

	/**
	 * Sanitize.
	 */
	public function sanitize() {
		$predicates = [
			'@on',
			'contains( @on, "tap:" )',
			'not( @tabindex ) or not( @role )',
			'not( self::button )',
			'not( self::a[ @href ] )',
		];

		$expression = sprintf(
			'//*[ %s ]',
			implode(
				' and ',
				array_map(
					static function ( $predicate ) {
						return "( $predicate )";
					},
					$predicates
				)
			)
		);

		$attributes = [
			'role'     => 'button',
			'tabindex' => '0',
		];

		/**
		 * Element.
		 *
		 * @var DOMElement $element
		 */
		foreach ( $this->dom->xpath->query( $expression ) as $element ) {
			foreach ( $attributes as $attr_name => $attr_value ) {
				if ( ! $element->hasAttribute( $attr_name ) ) {
					$element->setAttribute( $attr_name, $attr_value );
				}
			}
		}
	}
}

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