npx svelte-add@latest tailwindcss

package.json

 {
-  "name": "svekyll-next",
-  "version": "0.0.1",
-  "scripts": {
-    "dev": "svelte-kit dev",
-    "build": "svelte-kit build",
-    "preview": "svelte-kit preview",
-    "lint": "prettier --ignore-path .gitignore --check --plugin-search-dir=. . && eslint --ignore-path .gitignore .",
-    "format": "prettier --ignore-path .gitignore --write --plugin-search-dir=. ."
-  },
-  "devDependencies": {
-    "@sveltejs/kit": "next",
-    "eslint": "^7.32.0",
-    "eslint-config-prettier": "^8.3.0",
-    "eslint-plugin-svelte3": "^3.2.1",
-    "prettier": "^2.4.1",
-    "prettier-plugin-svelte": "^2.4.0",
-    "svelte": "^3.42.6"
-  },
-  "type": "module"
+	"name": "svekyll-next",
+	"version": "0.0.1",
+	"scripts": {
+		"dev": "svelte-kit dev",
+		"build": "svelte-kit build",
+		"preview": "svelte-kit preview",
+		"lint": "prettier --ignore-path .gitignore --check --plugin-search-dir=. . && eslint --ignore-path .gitignore .",
+		"format": "prettier --ignore-path .gitignore --write --plugin-search-dir=. ."
+	},
+	"devDependencies": {
+		"@sveltejs/kit": "next",
+		"eslint": "^7.32.0",
+		"eslint-config-prettier": "^8.3.0",
+		"eslint-plugin-svelte3": "^3.2.1",
+		"prettier": "^2.4.1",
+		"prettier-plugin-svelte": "^2.4.0",
+		"svelte": "^3.42.6",
+		"postcss": "^8.3.6",
+		"postcss-load-config": "^3.1.0",
+		"svelte-preprocess": "^4.9.4",
+		"autoprefixer": "^10.3.4",
+		"cssnano": "^5.0.8",
+		"tailwindcss": "^2.2.15"
+	},
+	"type": "module"
 }
\ No newline at end of file

postcss.config.cjs

+const tailwindcss = require("tailwindcss");
+const autoprefixer = require("autoprefixer");
+const cssnano = require("cssnano");
+
+const mode = process.env.NODE_ENV;
+const dev = mode === "development";
+
+const config = {
+	plugins: [
+        //Some plugins, like tailwindcss/nesting, need to run before Tailwind,
+        tailwindcss(),
+        //But others, like autoprefixer, need to run after,
+        autoprefixer(),
+        !dev && cssnano({
+			preset: "default",
+		})
+    ],
+};
+
+module.exports = config;
\ No newline at end of file

src/app.postcss

+/* Write your global styles here, in PostCSS syntax */
+@tailwind base;
+@tailwind components;
+@tailwind utilities
\ No newline at end of file

src/routes/__layout.svelte

+<script>import "../app.postcss";</script>
+<slot></slot>
\ No newline at end of file

svelte.config.js

+import preprocess from "svelte-preprocess";
 /** @type {import('@sveltejs/kit').Config} */
 const config = {
-	kit: {
+    kit: {
 		// hydrate the <div id="svelte"> element in src/app.html
 		target: '#svelte'
-	}
+	},
+
+    preprocess: [preprocess({
+        postcss: true
+    })]
 };
 
 export default config;

tailwind.config.cjs

+const config = {
+	mode: "jit",
+	purge: [
+		"./src/**/*.{html,js,svelte,ts}",
+	],
+	theme: {
+		extend: {},
+	},
+	plugins: [],
+};
+
+module.exports = config;