okumurakengo/index.css

## index.css
/* src/index.css */
body {
    background: yellow;
}

## index.html
<!DOCTYPE html>
<meta charset="UTF-8">
<title>Document</title>
<script src="dist/main.js"></script>

## index.js
// src/index.js
import "./index.css"
console.log(123)

## package.json
{
  "devDependencies": {
    "css-loader": "^3.2.0",
    "style-loader": "^1.0.0",
    "webpack": "^4.41.2",
    "webpack-cli": "^3.3.10"
  },
  "sideEffects": [
    "src/index.css"
  ]
}

## webpack.config.js
module.exports = {
    mode: "production",
    module: {
        rules: [
            {
                test: /\.css$/i,
                use: ["style-loader", "css-loader"],
            },
        ],
    },
};
	<!DOCTYPE html>
	<meta charset="UTF-8">
	<title>Document</title>
	<script src="dist/main.js"></script>
	{
	"devDependencies": {
	"css-loader": "^3.2.0",
	"style-loader": "^1.0.0",
	"webpack": "^4.41.2",
	"webpack-cli": "^3.3.10"
	},
	"sideEffects": [
	"src/index.css"
	]
	}
	module.exports = {
	mode: "production",
	module: {
	rules: [
	{
	test: /\.css$/i,
	use: ["style-loader", "css-loader"],
	},
	],
	},
	};