Fix build errors

Author: hacknug

package.json

@@ -36,6 +36,8 @@
   },
   "devDependencies": {
     "@fullhuman/postcss-purgecss": "^1.2.0",
+    "autoprefixer": "^9.5.1",
+    "cssnano": "^4.1.10",
     "eslint": "^5.16.0",
     "eslint-config-standard": "^12.0.0",
     "eslint-plugin-import": "^2.17.2",