<div class="container">
  <form action="/action_page.html">
    <label for="usrname">Username</label>
    <input type="text" id="usrname" name="usrname" required>

    <label for="psw">Password</label>
  <  input type="password" id="psw" name="psw" pattern="(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{8,}" title="Must contain at least one number and one uppercase and lowercase letter, and at least 8 or more characters" required>

    <input type="submit" value="Submit">
  </form>
</div>

<div id="message">
  <h3>Password must contain the following:</h3>
  <p id="letter" class="invalid">A <b>lowercase</b> letter</p>
  <p id="capital" class="invalid">A <b>capital (uppercase)</b> letter</p>
  <p id="number" class="invalid">A <b>number</b></p>
  <p id="length" class="invalid">Minimum <b>8 characters</b></p>
</div>

<script>
var myInput = document.getElementById("psw");
var letter = document.getElementById("letter");
var capital = document.getElementById("capital");
var number = document.getElementById("number");
var length = document.getElementById("length");

// When the user clicks on the password field, show the message box
myInput.onfocus = function() {
  document.getElementById("message").style.display = "block";
}

// When the user clicks outside of the password field, hide the message box
myInput.onblur = function() {
  document.getElementById("message").style.display = "none";
}

// When the user starts to type something inside the password field
myInput.onkeyup = function() {
  // Validate lowercase letters
  var lowerCaseLetters = /[a-z]/g;
  if(myInput.value.match(lowerCaseLetters)) {
    letter.classList.remove("invalid");
    letter.classList.add("valid");
  } else {
    letter.classList.remove("valid");
    letter.classList.add("invalid");
}

  // Validate capital letters
  var upperCaseLetters = /[A-Z]/g;
  if(myInput.value.match(upperCaseLetters)) {
    capital.classList.remove("invalid");
    capital.classList.add("valid");
  } else {
    capital.classList.remove("valid");
    capital.classList.add("invalid");
  }

  // Validate numbers
  var numbers = /[0-9]/g;
  if(myInput.value.match(numbers)) {
    number.classList.remove("invalid");
    number.classList.add("valid");
  } else {
    number.classList.remove("valid");
    number.classList.add("invalid");
  }

  // Validate length
  if(myInput.value.length >= 8) {
    length.classList.remove("invalid");
    length.classList.add("valid");
  } else {
    length.classList.remove("valid");
    length.classList.add("invalid");
  }
}
</script>