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index.css

* {
	box-sizing: border-box;
}

html {
	font-family: sans-serif;
}

index.tsx

/*
 * Here's an example of a React Component. However,
 * you can use another framework, or just import 
 * libraries like jQuery, lodash, d3, etc.
 */
import * as ReactDOM from "react-dom";
import * as React from "react";
import "./index.css";

function Counter() {
	const [count, setCount] = React.useState(0);
	
	return (
		<div>
			<p>Count: {count}</p>
			<button onClick={() => setCount(count + 1)}>Increment</button>
		</div>
	);
}

ReactDOM.render(
	<Counter />,
	document.body
);

A comment.