<img> srcset sizes calculation in different wordpress themes

functions.php

/* https://www.smashingmagazine.com/2015/12/responsive-images-in-wordpress-core/ */

function adjust_image_sizes_attr( $sizes, $size ) {
   $sizes = '(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px';
   return $sizes;
}
add_filter( 'wp_calculate_image_sizes', 'adjust_image_sizes_attr', 10 , 2 );

template-functions.php

/**
 * Add custom sizes attribute to responsive image functionality for post thumbnails.
 *
 * @origin Twenty Nineteen 1.0
 *
 * @param array $attr  Attributes for the image markup.
 * @return string Value for use in post thumbnail 'sizes' attribute.
 */
function twentynineteen_post_thumbnail_sizes_attr( $attr ) {

	if ( is_admin() ) {
		return $attr;
	}

	if ( ! is_singular() ) {
		$attr['sizes'] = '(max-width: 34.9rem) calc(100vw - 2rem), (max-width: 53rem) calc(8 * (100vw / 12)), (min-width: 53rem) calc(6 * (100vw / 12)), 100vw';
	}

	return $attr;
}
add_filter( 'wp_get_attachment_image_attributes', 'twentynineteen_post_thumbnail_sizes_attr', 10, 1 );

• https://via.studio/journal/optimizing-your-theme-for-wordpress-4-4s-responsive-images
• https://pineco.de/controlling-responsive-image-sizes-in-wordpress/
• Apply a filter to the wp_calculate_image_sizes to enable sizes attribute for srcsets (for largo theme) WPBuddy/largo#1577
• "WP adds proper srcset attribute, but the sizes attribute is pretty useless" WordPress/gutenberg#6177