ลบสีขอบที่กำหนดไว้ก่อนหน้า

function validate() {
	var username = document.getElementById("username").value;
	var password = document.getElementById("password").value;

	if (username == "") {

		document.getElementById("message").innerHTML = "USERNAME CANNOT BE EMPTY";
		document.getElementById("username").style.borderColor = "red";
		return false;
	}
	if (password == "") {


		document.getElementById("message").innerHTML = "PASSWORD CANNOT BE EMPTY";
		document.getElementById("password").style.borderColor = "red";
		return false;
	}
}

#username:focus {
	background-color: yellow;
	border-color: green;
}

#password:focus {
	background-color: yellow;
	border-color: green;
}

#message {
	color: red;
}

<form onsubmit=" return validate()">
    LOGIN:-
    <br>
    <input id="username" type="text" name="username" placeholder="USERNAME">
    <br>
    <input id="password" type="password" name="password" placeholder="PASSWORD">
    <br>
    <input type="submit" value="SUBMIT">
    <p id="message">
</form>

function validate() {
    var username = document.getElementById("username").value;
    var password = document.getElementById("password").value;

    if (username == "") {
        document.getElementById("message").innerHTML = "USERNAME CANNOT BE EMPTY";
        document.getElementById("username").classList.add("invalidInput");
        return false;
    } else {
        document.getElementById("username").classList.remove("invalidInput")
    }
    if (password == "") {
        document.getElementById("message").innerHTML = "PASSWORD CANNOT BE EMPTY";
        document.getElementById("password").classList.add("invalidInput")
        return false;
    } else {
        document.getElementById("password").classList.remove("invalidInput")
    }
}

#username:focus {
    background-color: yellow;
    border-color: green;
}

#password:focus {
    background-color: yellow;
    border-color: green;
}

.invalidInput {
    border-color: red; 
}

#message {
    color: red;
}

<form onsubmit=" return validate()">
    LOGIN:-
    <br />
    <input id="username" type="text" name="username" placeholder="USERNAME" />
    <br />
    <input id="password" type="password" name="password" placeholder="PASSWORD" />
    <br />
    <input type="submit" value="SUBMIT" />
    <p id="message"></p>
</form>

function validate() {
  var username = document.getElementById("username").value;
  var password = document.getElementById("password").value;

  if (username == "") {

    document.getElementById("message").innerHTML = "USERNAME CANNOT BE EMPTY";
    //document.getElementById("username").style.borderColor = "red";
    return false;
  }
  if (password == "") {


    document.getElementById("message").innerHTML = "PASSWORD CANNOT BE EMPTY";
    //document.getElementById("password").style.borderColor = "red";
    return false;
  }

}

#username:focus{
  background-color: yellow;
  border-color: green;
}

#username:invalid{
  background-color: none;
  border-color: red;
}

#password:focus{
  background-color: yellow;
  border-color: green;
}
#password:invalid{
  background-color: none;
  border-color: red;
}


#message {
  color: red;
}

<form onsubmit=" return validate()">
  LOGIN:-
  <br>
  <input id="username" type="text" name="username" placeholder="USERNAME" required>
  <br>
  <input id="password" type="password" name="password" placeholder="PASSWORD" required>
  <br>
  <input type="submit" value="SUBMIT">
  <p id="message">

</form>

function retainColor(ele){
  ele.style.borderColor = "inherit";
  document.getElementById("message").innerHTML = "";
}
function validate() {
  var username = document.getElementById("username").value;
  var password = document.getElementById("password").value;

  if (username == "") {

    document.getElementById("message").innerHTML = "USERNAME CANNOT BE EMPTY";
    document.getElementById("username").classList.add("error");
    return false;
  }else{
    document.getElementById("username").classList.remove("error");
  }
  if (password == "") {
    document.getElementById("message").innerHTML = "PASSWORD CANNOT BE EMPTY";
    document.getElementById("password").classList.add("error");
    return false;
  }else{
    document.getElementById("password").classList.remove("error");
  }

}

#username:focus {
  background-color: yellow;
  border-color: green;
}

#password:focus {
  background-color: yellow;
  border-color: green;
}
.error {
  border-color: red;
}

.error {
  border-color: red;
}

#message {
  color: red;
}

<form onsubmit=" return validate()">
  LOGIN:-
  <br>
  <input id="username" type="text" onkeyup="retainColor(this)" name="username" placeholder="USERNAME">
  <br>
  <input id="password" type="password" onkeyup="retainColor(this)" name="password" placeholder="PASSWORD">
  <br>
  <input type="submit" value="SUBMIT">
  <p id="message">

</form>