temps.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>
    <link rel="stylesheet" href="styles.css" />
  </head>
  <body>
    <h1 id="greet">Temperature conversion</h1>

    <div class="tempWrapper">
      <input
        class="tempInput"
        type="text"
        onkeyup="ctof()"
        name="tempc"
        id="tempc"
        placeholder="Celcius"
        id="Celcius"
      />
      <span>Celcius</span>
    </div>

    <div class="tempWrapper">
      <input
        class="tempInput"
        type="text"
        onkeyup="ftoc()"
        name="tempf"
        id="tempf"
        placeholder="Fahrenheit"
      />
      <span>Fahrenheit</span>
    </div>

    <script src="./temp.js"></script>
  </body>
</html>