This page isn’t working…If the problem continues, contact the site owner. HTTP ERROR 405

I am making this simple login form using html css and js. And I am receiving this error when i click the login button: This page isn’t working…If the problem continues, contact the site owner. HTTP ERROR 405.
Below are the codes:

JAVASCRIPT:

function validate(){
  var username = document.getElementById(username).value
  var password = document.getElementById(password).value
  if(username == "judge1" && password=="1234") {
    alert("Login Successful");
    return false;
  }
  else{
    alert("Login Failed");
  }
}

HTML:

<form class="inputs" action="/" method="POST">
        <input type="text" class="username" placeholder="Username" id="username"> <br/>
        <input type="password" class="password" placeholder="Password" id="password"> <br/>
        <input type="submit" value="Login" onclick="validate()">
    </form>

I tried googling it but I’m having a hard time in finding the solution. I am still new in this so please help.