Preview:
<!-- <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        function helloworld(){
            document.write("hello world");
        }
    </script>
    <form>
        <input type="submit" onclick = helloworld()>
    </form>
</body>
</html> -->

<!-- <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        function validateform(){
            var name=document.getElementById("name").value;
            var roll=document.getElementById("roll").value;
            if(name==="" || roll===""){
                alert("name and roll required");
                return false
            }
            return true;
            
        }
    </script>

</head>
<body>
    <form >
        <label for ="name">Name</label>
        <input type="txt" id="name">
        <label for ="roll" >roll</label>
        <input type="number" id ="roll">
        <input type="submit"onclick=validateform());
    </form>
    
</body>
</html> -->
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