index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <h1>EVEN OR ODD</h1>
    <style>
        h1 {text-align: center;}
    </style>

<form onsubmit="dosum();return false">
    num: <input id="num1" type="number" required placeholder="Enter a Number" >
    <br>
    <br>
    <button type="submit">submit</button>
    </form>
    <h3 id="result"></h3>

    <script>
        function dosum() {
            let firstNumber = Number(document.querySelector("#num1").value)
            let resultElement = document.querySelector("#result")
           if (firstNumber % 2 === 0) {
            resultElement.innerHTML = "Your number is Even";
           }
           else { resultElement.innerHTML = "Your number is ODD";}
        }
    </script>
</body>
</html>