index.html

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
</head>
<body>
  <h1>Bem-vindo(a)!</h1>

  <script>
    (function () {
      fetch('./api.php')
        .then(response => {
          if (response.redirected === true) {
            location.href = response.url
          }
        })
    })()
  </script>
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</html>