Textbook: Use wp_kses_post rather than wp_filter_post_kses. See 5019-wpcom-themes.

textbook/components/features/testimonials/archive-jetpack-testimonial.php

@@ -35,7 +35,7 @@ get_header(); ?>
 
 				<article class="hentry">
 					<div class="entry-content">
-						<?php echo convert_chars( convert_smilies( wptexturize( stripslashes( wp_filter_post_kses( addslashes( $jetpack_options['page-content'] ) ) ) ) ) ); ?>
+						<?php echo convert_chars( convert_smilies( wptexturize( wp_kses_post( $jetpack_options['page-content'] ) ) ) ); ?>
 					</div>
 				</article>
 
@@ -67,4 +67,4 @@ get_header(); ?>
 	<?php endif;
 		wp_reset_postdata(); ?>
 
-<?php get_footer(); ?>
+<?php get_footer(); ?>

textbook/components/features/testimonials/display-testimonials.php

@@ -23,7 +23,7 @@
 					?>
 				</h2>
 				<div class="testimonials-content">
-					<?php echo convert_chars( convert_smilies( wptexturize( stripslashes( wp_filter_post_kses( addslashes( $jetpack_options['page-content'] ) ) ) ) ) ); ?>
+					<?php echo convert_chars( convert_smilies( wptexturize( wp_kses_post( $jetpack_options['page-content'] ) ) ) ); ?>
 				</div>
 			</header><!-- .entry-header -->
 
@@ -39,4 +39,4 @@
 			</div>
 		</div>
 	</div><!-- .testimonials -->
-<?php endif; ?>
+<?php endif; ?>

textbook/style.css

@@ -4,7 +4,7 @@ Theme URI: http://wordpress.com/theme/textbook/
 Author: Automattic
 Author URI: https://wordpress.com/themes/
 Description: TextBook is a theme for colleges, schools and organizations focused on education.
-Version: 1.1.5-wpcom
+Version: 1.1.6-wpcom
 License: GNU General Public License v2 or later
 License URI: http://www.gnu.org/licenses/gpl-2.0.html
 Text Domain: textbook