Definitely postcss. separated out the commands for ease of use:
"scripts": {
"start": "eleventy --serve & postcss src/css/style.css --o _tmp/style.css --watch",
"build": "ELEVENTY_PRODUCTION=true eleventy && NODE_ENV=production postcss src/css/style.css --o _site/style.css",
"buildnetlify": "npm-run-all build:eleventy build:postcss",
"build:eleventy": "eleventy",
"build:postcss": "postcss src/css/style.css --o _site/style.css"
},
"keywords": [],
"author": "",
"license": "ISC",
"dependencies": {
"@11ty/eleventy": "^0.11.1",
"npm-run-all": "^4.1.5",
"postcss": "^8.2.4"
},
"devDependencies": {
"alpinejs": "^2.8.0",
"autoprefixer": "^10.2.4",
"tailwindcss": "^2.0.2"
}
and logs.
....
3:15:18 PM: $ npm run buildnetlify
3:15:19 PM: > limoncello@1.0.0 buildnetlify /opt/build/repo
3:15:19 PM: > npm-run-all build:eleventy build:postcss
3:15:19 PM: > limoncello@1.0.0 build:eleventy /opt/build/repo
3:15:19 PM: > eleventy
3:15:19 PM: Writing _site/index.html from ./src/index.njk.
3:15:19 PM: Copied 1 file / Wrote 1 file in 0.07 seconds (v0.11.1)
3:15:20 PM: > limoncello@1.0.0 build:postcss /opt/build/repo
3:15:20 PM: > postcss src/css/style.css --o _site/style.css
3:15:20 PM: sh: 1: postcss: not found
....