Fix CSS fallback

Turn off `mergeLonghand` because it combines `padding-*`,
breaking fallback styles.

cssnano.config.js

@@ -0,0 +1,10 @@
+module.exports = {
+    preset: [
+        'default',
+        // Turn off `mergeLonghand` because it combines `padding-*` and `margin-*`,
+        // breaking fallback styles.
+        // https://github.com/cssnano/cssnano/issues/1163
+        // https://github.com/cssnano/cssnano/issues/1192
+        { mergeLonghand: false }
+    ]
+};