Addresses WordPress#50. Add custom sizes attribute to body images. Re…

…lies on Gutenberg #11973.

inc/template-functions.php

@@ -134,6 +134,36 @@ function twentynineteen_post_thumbnail_sizes_attr( $attr ) {
 }
 add_filter( 'wp_get_attachment_image_attributes', 'twentynineteen_post_thumbnail_sizes_attr', 10, 1 );
 
+/**
+ * Add custom sizes attribute to responsive image functionality for regular images.
+ *
+ * @origin Twenty Nineteen 1.0
+ *
+ * @param array $image_attr  Attributes for the image markup.
+ * @param array $block_attr  Attributes for the containing block.
+ * @return string Value for use in post thumbnail 'sizes' attribute.
+ */
+function twentynineteen_calculate_image_sizes_attr( $image_attr, $block_attr ) {
+
+	if ( is_admin() ) {
+		return $image_attr;
+	}
+
+	$align = $block_attr[align];
+
+	if ( '' === $align ) {
+		$image_attr['sizes'] = '(max-width: 34.9rem) calc(100vw - 2rem), (max-width: 53rem) calc(8 * (100vw / 12)), (min-width: 53rem) calc(6 * (100vw / 12)), 100vw';
+	} elseif ( 'wide' === $align ) {
+		$image_attr['sizes'] = '(max-width: 34.9rem) calc(100vw - 2rem), (min-width: 35rem) 80vw, 100vw';
+	} elseif ( 'full' === $align ) {
+		// Use default.
+	}
+
+	return $image_attr;
+
+}
+add_filter( 'render_block_core_image_tag_attributes', 'twentynineteen_calculate_image_sizes_attr', 10, 2 );
+
 /**
  * Returns the size for avatars used in the theme.
  */