Source

File: wp-includes/comment-template.php

function get_comment_class( $class = '', $comment_id = null, $post_id = null ) {
	global $comment_alt, $comment_depth, $comment_thread_alt;

	$classes = array();

	$comment = get_comment( $comment_id );
	if ( ! $comment ) {
		return $classes;
	}

	// Get the comment type (comment, trackback).
	$classes[] = ( empty( $comment->comment_type ) ) ? 'comment' : $comment->comment_type;

	// Add classes for comment authors that are registered users.
	$user = $comment->user_id ? get_userdata( $comment->user_id ) : false;
	if ( $user ) {
		$classes[] = 'byuser';
		$classes[] = 'comment-author-' . sanitize_html_class( $user->user_nicename, $comment->user_id );
		// For comment authors who are the author of the post.
		$post = get_post( $post_id );
		if ( $post ) {
			if ( $comment->user_id === $post->post_author ) {
				$classes[] = 'bypostauthor';
			}
		}
	}

	if ( empty( $comment_alt ) ) {
		$comment_alt = 0;
	}
	if ( empty( $comment_depth ) ) {
		$comment_depth = 1;
	}
	if ( empty( $comment_thread_alt ) ) {
		$comment_thread_alt = 0;
	}

	if ( $comment_alt % 2 ) {
		$classes[] = 'odd';
		$classes[] = 'alt';
	} else {
		$classes[] = 'even';
	}

	$comment_alt++;

	// Alt for top-level comments.
	if ( 1 == $comment_depth ) {
		if ( $comment_thread_alt % 2 ) {
			$classes[] = 'thread-odd';
			$classes[] = 'thread-alt';
		} else {
			$classes[] = 'thread-even';
		}
		$comment_thread_alt++;
	}

	$classes[] = "depth-$comment_depth";

	if ( ! empty( $class ) ) {
		if ( ! is_array( $class ) ) {
			$class = preg_split( '#\s+#', $class );
		}
		$classes = array_merge( $classes, $class );
	}

	$classes = array_map( 'esc_attr', $classes );

	/**
	 * Filters the returned CSS classes for the current comment.
	 *
	 * @since 2.7.0
	 *
	 * @param string[]    $classes    An array of comment classes.
	 * @param string      $class      A comma-separated list of additional classes added to the list.
	 * @param int         $comment_id The comment ID.
	 * @param WP_Comment  $comment    The comment object.
	 * @param int|WP_Post $post_id    The post ID or WP_Post object.
	 */
	return apply_filters( 'comment_class', $classes, $class, $comment->comment_ID, $comment, $post_id );
}