comments.php

<?php
/**
 * The template for displaying comments
 *
 * The area of the page that contains both current comments
 * and the comment form.
 *
 * @package Twenty_Fifteen_AMP
 */

// Include regular comments template if not AMP.
if ( ! twentyfifteen_is_amp() ) {
	require trailingslashit( get_template_directory() ) . basename( __FILE__ );
	return;
}

/*
 * If the current post is protected by a password and
 * the visitor has not yet entered the password we will
 * return early without loading the comments.
 */
if ( post_password_required() ) {
	return;
}
?>

<div id="comments" class="comments-area">

	<?php if ( have_comments() ) : ?>
		<h2 class="comments-title">
			<?php
			$comments_number = get_comments_number();
			if ( '1' === $comments_number ) {
				/* translators: %s: post title */
				echo esc_html( sprintf( _x( 'One thought on &ldquo;%s&rdquo;', 'comments title', 'twentyfifteen' ), get_the_title() ) );
			} else {
				echo esc_html( sprintf(
					/* translators: 1: number of comments, 2: post title */
					_nx(
						'%1$s thought on &ldquo;%2$s&rdquo;',
						'%1$s thoughts on &ldquo;%2$s&rdquo;',
						$comments_number,
						'comments title',
						'twentyfifteen'
					),
					number_format_i18n( $comments_number ),
					get_the_title()
				) );
			}
			?>
		</h2>

		<?php twentyfifteen_comment_nav(); ?>

		<amp-live-list
			id="amp-live-comments-list-<?php the_ID(); ?>"
			<?php echo ( 'asc' === get_option( 'comment_order' ) ) ? ' sort="ascending" ' : ''; ?>
			data-poll-interval="<?php echo esc_attr( 15000 ); ?>"
			data-max-items-per-page="<?php echo esc_attr( get_option( 'page_comments' ) ? get_option( 'comments_per_page' ) : 10000 ); ?>"
		>
			<ol items class="comment-list">
				<?php
					wp_list_comments( array(
						'style'       => 'ol',
						'short_ping'  => true,
						'avatar_size' => 56,
					) );
				?>
			</ol><!-- .comment-list -->

			<?php twentyfifteen_comment_nav( array( 'is_amp_live_list_pagination' => true ) ); ?>

			<p update>
				<button class="button" on="tap:amp-live-comments-list-<?php the_ID(); ?>.update"><?php esc_html_e( 'New comment(s)', '_s' ); ?></button>
			</p>
		</amp-live-list>

	<?php endif; // End of have_comments() check. ?>


	<?php if ( ! comments_open() && get_comments_number() && post_type_supports( get_post_type(), 'comments' ) ) : ?>
		<p class="no-comments"><?php esc_html_e( 'Comments are closed.', 'twentyfifteen' ); ?></p>
	<?php endif; ?>

	<?php comment_form(); ?>

</div><!-- .comments-area -->

functions.php

<?php
/**
 * Twenty Fifteen AMP Functions.
 *
 * @package Twenty_Fifteen_AMP
 */

/**
 * Return whether AMP endpoint.
 *
 * @return bool Whether AMP.
 */
function twentyfifteen_is_amp() {
	return function_exists( 'is_amp_endpoint' ) && is_amp_endpoint();
}

/**
 * Determine whether the sidebar (secondary) is present.
 *
 * This replaces the conditional that was in the parent theme's sidebar.php.
 *
 * @return bool Whether sidebar is present.
 */
function twentyfifteen_has_secondary_sidebar() {
	return has_nav_menu( 'primary' ) || has_nav_menu( 'social' ) || is_active_sidebar( 'sidebar-1' );
}

/**
 * Add pagination reference point attribute for amp-live-list when theme supports AMP.
 *
 * This is used by the navigation_markup_template filter in the comments template.
 *
 * @link https://www.ampproject.org/docs/reference/components/amp-live-list#pagination
 * @todo AMP should allow nav instead of element with [pagination] attribute.
 *
 * @param string $markup Navigation markup.
 * @return string Markup.
 */
function twentyfifteen_add_amp_live_list_pagination_attribute( $markup ) {
	return preg_replace( '/(\s*<[a-z0-9_-]+)/i', '$1 pagination ', $markup, 1 );
}

/**
 * Display navigation to next/previous comments when applicable.
 *
 * As of WordPress 4.4 we can now use `the_comments_navigation()` here.
 *
 * @todo AMP should allow a nav in place of element with [pagination] attribute.
 * @param array $args {
 *     Args.
 *
 *     @type bool $is_amp_live_list_pagination Is the pagination for amp-live-list.
 * }
 */
function twentyfifteen_comment_nav( $args = array( 'is_amp_live_list_pagination' => false ) ) {
	if ( ! empty( $args['is_amp_live_list_pagination'] ) ) {
		add_filter( 'navigation_markup_template', 'twentyfifteen_add_amp_live_list_pagination_attribute' );
	}
	the_comments_navigation();
	if ( ! empty( $args['is_amp_live_list_pagination'] ) ) {
		remove_filter( 'navigation_markup_template', 'twentyfifteen_add_amp_live_list_pagination_attribute' );
	}
}

/**
 * Add AMP theme support.
 */
function twentyfifteen_amp_after_setup_theme() {
	if ( defined( 'AMP__VERSION' ) && version_compare( strtok( AMP__VERSION, '-' ), '1.0' ) ) {
		add_action( 'admin_notices', 'twentyfifteen_amp_version_required_admin_notoce' );
		return;
	}

	add_post_type_support( 'page', 'amp' );

	// To opt-in for paired mode instead of Native AMP, add 'template_dir' => './' to the following amp theme support args.
	add_theme_support( 'amp', array(
		'comments_live_list' => true,
	) );
}
add_action( 'after_setup_theme', 'twentyfifteen_amp_after_setup_theme' );

/**
 * Show notice when the required AMP version is not installed.
 */
function twentyfifteen_amp_version_required_admin_notoce() {
	?>
	<div class="notice notice-warning">
		<p>
			<?php
			echo wp_kses_post(
				sprintf(
					/* translators: %1$s is the current plugin version */
					__( 'The Twenty Fifteen Amplified theme requires AMP plugin v1.0 (76e1657), but you have %1$s. Please update to use with <code>amp</code> theme support.', 'twentyfifteen-amp' ),
					esc_html( AMP__VERSION )
				)
			);
			?>
		</p>
	</div>
	<?php
}

/**
 * Enqueue CSS from parent theme and this child theme.
 *
 * @see twentyfifteen_scripts()
 */
function twentyfifteen_amp_scripts() {
	$parent_stylesheet      = wp_styles()->registered[ get_template() . '-style' ];
	$parent_stylesheet->src = get_template_directory_uri() . '/style.css';
	$parent_stylesheet->ver = wp_get_theme( get_template() )->get( 'Version' );

	wp_enqueue_style(
		get_stylesheet() . '-style',
		get_stylesheet_directory_uri() . '/style.css',
		array( get_template() . '-style' ),
		wp_get_theme( get_stylesheet() )->get( 'Version' )
	);
}
add_action( 'wp_enqueue_scripts', 'twentyfifteen_amp_scripts', 20 );

style.css

/*
Theme Name: Twenty Fifteen Amplified
Author: XWP
Description: AMP child theme of Twenty Fifteen.
Version: 0.1
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Template: twentyfifteen
*/

html[amp] #commentform [submit-success] p {
	margin-top: 1em;
}