JSFiddle - React, Tailwind, and code Playground

HTML

<form action="/authorisation.php">
    <input required name="login" placeholder="Логин">
    <input required type="password" name="password" placeholder="Пароль">
    <button>Войти!</button>
</form>

CSS

input, button {
    margin-top: 10px; 
    display: block;
}

JavaScript

var input = document.querySelector("input");
var form = document.querySelector("form");
input.onkeydown = function(e)
{
    var symbol = String.fromCharCode(e.keyCode);
    if((/[A-Z0-9]/).test(symbol) && input.value.length < 10)
    {
        setTimeout(function()
                   {
                       input.value = input.value.replace(/[^a-zA-Z0-9]/gi, "");
                   }, 1);
        return true;
    }
    if(e.keyCode == 8 || e.keyCode == 37 || e.keyCode == 39)
    {
        return true;
    }
    return false;
};
form.onsubmit = function()
{
    if(input.value.length >= 5)
    {
        return true;
    }
    else
    {
        alert("Длина значения должна быть больше или равна пяти!");
        return false;
    }
}