WordPress default and custom classes are gone in

I assume you’re using Reader mode? If so, body classes are added differently. You can put this code in your theme’s functions.php to add your custom class:

add_filter(
	'amp_post_template_data',
	static function ( $data ) {
		if ( is_singular() ) {
			$data['body_class'] .= ' ' . get_post_meta( get_the_ID(), 'page_body_class', true );
		}
		return $data;
	}
);

In v2.0 the AMP plugin will allow selecting an entire AMP-compatible theme to be used in Reader mode, for example Twenty Twenty. In that case the normal body_class function and filters will apply. You can use this filter to avoid modifying your templates. When that happens, this would be the preferred code for AMP and non-AMP alike, placed in a custom plugin:

add_filter( 
	'body_class', 
	static function ( $body_class ) {
		if ( is_singular() ) {
			$body_class[] = get_post_meta( get_the_ID(), 'page_body_class', true );
		}
		return $body_class;
	} 
);

But for the time being, use the amp_post_template_data filter in your theme’s functions.php.

• This reply was modified 4 years, 7 months ago by Weston Ruter.