add some packages

node_modules/clean-css/lib/optimizer/level-2/properties/understandable.js

@@ -0,0 +1,15 @@
+var sameVendorPrefixes = require('./vendor-prefixes').same;
+
+function understandable(validator, value1, value2, _position, isPaired) {
+  if (!sameVendorPrefixes(value1, value2)) {
+    return false;
+  }
+
+  if (isPaired && validator.isVariable(value1) !== validator.isVariable(value2)) {
+    return false;
+  }
+
+  return true;
+}
+
+module.exports = understandable;