When you want to customize html template. You can customize template by create special file named pages/_document.tsx.
The default template is as follows.
Props is empty object. Children are your page component will injected.
const Document = (_: {}, children: Component[]) => {
return (
<html lang="en">
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>Document</title>
</head>
<body>
{children}
</body>
</html>
);
}