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HTML 

              
                <div class="container">
  <h2 class="title">Create a new account</h2>
  <form action="#" class="form">

    <div class="input-group">
      <label for="username" class="label">Username</label>
      <input id="username" placeholder="webcrunch" type="text" class="input">
      <span class="error-message"></span>
      <svg class="icon icon-success hidden hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M10 18a8 8 0 100-16 8 8 0 000 16zm3.707-9.293a1 1 0 00-1.414-1.414L9 10.586 7.707 9.293a1 1 0 00-1.414 1.414l2 2a1 1 0 001.414 0l4-4z" clip-rule="evenodd" />
      </svg>

      <svg class="icon icon-error hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M18 10a8 8 0 11-16 0 8 8 0 0116 0zm-7 4a1 1 0 11-2 0 1 1 0 012 0zm-1-9a1 1 0 00-1 1v4a1 1 0 102 0V6a1 1 0 00-1-1z" clip-rule="evenodd" />
      </svg>
    </div>

    <div class="input-group">
      <label for="email" class="label">Email</label>
      <input id="email" type="email" class="input" autocomplete placeholder="andy@web-crunch.com">
      <span class="error-message"></span>
      <svg class="icon icon-success hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M10 18a8 8 0 100-16 8 8 0 000 16zm3.707-9.293a1 1 0 00-1.414-1.414L9 10.586 7.707 9.293a1 1 0 00-1.414 1.414l2 2a1 1 0 001.414 0l4-4z" clip-rule="evenodd" />
      </svg>

      <svg class="icon icon-error hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M18 10a8 8 0 11-16 0 8 8 0 0116 0zm-7 4a1 1 0 11-2 0 1 1 0 012 0zm-1-9a1 1 0 00-1 1v4a1 1 0 102 0V6a1 1 0 00-1-1z" clip-rule="evenodd" />
      </svg>
    </div>

    <div class="input-group">
      <label for="password" class="label">Password</label>
      <input id="password" type="password" class="input">
      <span class="error-message"></span>
      <svg class="icon icon-success hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M10 18a8 8 0 100-16 8 8 0 000 16zm3.707-9.293a1 1 0 00-1.414-1.414L9 10.586 7.707 9.293a1 1 0 00-1.414 1.414l2 2a1 1 0 001.414 0l4-4z" clip-rule="evenodd" />
      </svg>

      <svg class="icon icon-error hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M18 10a8 8 0 11-16 0 8 8 0 0116 0zm-7 4a1 1 0 11-2 0 1 1 0 012 0zm-1-9a1 1 0 00-1 1v4a1 1 0 102 0V6a1 1 0 00-1-1z" clip-rule="evenodd" />
      </svg>
    </div>

    <div class="input-group">
      <label for="password_confirmation" class="label">Password Confirmation</label>
      <input id="password_confirmation" type="password" class="input">
      <span class="error-message"></span>
      <svg class="icon icon-success hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M10 18a8 8 0 100-16 8 8 0 000 16zm3.707-9.293a1 1 0 00-1.414-1.414L9 10.586 7.707 9.293a1 1 0 00-1.414 1.414l2 2a1 1 0 001.414 0l4-4z" clip-rule="evenodd" />
      </svg>

      <svg class="icon icon-error hidden" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
        <path fill-rule="evenodd" d="M18 10a8 8 0 11-16 0 8 8 0 0116 0zm-7 4a1 1 0 11-2 0 1 1 0 012 0zm-1-9a1 1 0 00-1 1v4a1 1 0 102 0V6a1 1 0 00-1-1z" clip-rule="evenodd" />
      </svg>
    </div>

    <input type="submit" class="button" value="Create account">

  </form>
</div>

<div class="promo">👋 Check out my <a href="https://web-crunch.com" target="_blank">blog</a> or my <a href="https://youtube.com/webcrunch">YouTube channel</a> to learn more about how I made this.</div>
!

CSS 

              
                * {
	box-sizing: border-box;
}

body {
  background-color: blueviolet;  
}

.title {
  margin-bottom: 2rem;
}

.hidden {
  display: none;
}

.icon {
  width: 24px;
  height: 24px;
  position: absolute;
  top: 32px; 
  right: 5px;
  pointer-events: none;
  z-index: 2;
  
  &.icon-success {
    fill: green;
  }
  
  &.icon-error {
    fill: red;
  }
}

.container {
  max-width: 460px;
  margin: 3rem auto;
  padding: 3rem;
  border: 1px solid #ddd;
  border-radius: .25rem;
  background-color: white;
  box-shadow: 0 20px 25px -5px rgba(0, 0, 0, 0.1), 0 10px 10px -5px rgba(0, 0, 0, 0.04);
}

.label {
  font-weight: bold;
  display: block;
  color: #333;
  margin-bottom: .25rem;
  color: #2d3748;
}

.input {
  appearance: none;
  display: block;
  width: 100%;
  color: #2d3748;
  border: 1px solid #cbd5e0;
  line-height: 1.25;
  background-color: white;
  padding: .65rem .75rem;
  border-radius: 0.25rem;
  box-shadow: inset 0 2px 4px 0 rgba(0, 0, 0, 0.06);
  
  &::placeholder {
    color: #a0aec0;
  }
  
  &.input-error {
    border: 1px solid red;
    
    &:focus {
      border: 1px solid red;
    }
  }
  
  &:focus {
    outline: none;
    border: 1px solid #a0aec0;
    box-shadow: 0 1px 3px 0 rgba(0, 0, 0, 0.1), 0 1px 2px 0 rgba(0, 0, 0, 0.06);
    background-clip: padding-box;
  }
}

.input-group {
  margin-bottom: 2rem;
  position: relative;
}

.error-message {
  font-size: .85rem;
  color: red;
}

.button {
  background-color: blueviolet;
  padding: 1rem 2rem;
  border: none;
  border-radius: .25rem;
  color: white;
  font-weight: bold;
  display: block;
  width: 100%;
  text-align: center;
  cursor: pointer;
  
  &:hover {
    filter: brightness(110%);
  }
}


.promo {
  color: white;
  opacity: .75;
  margin: 1rem auto;
  max-width: 460px;
  background: rgba(255, 255, 255, .2);
  padding: 20px;
  border-radius: .25rem;
    
  a {
    color: white;
  }
}
!

JS 

              
                class FormValidator {
  constructor(form, fields) {
    this.form = form
    this.fields = fields
  }  
  
  initialize() {
    this.validateOnEntry()
    this.validateOnSubmit()
  }
  
  validateOnSubmit() {
    let self = this
    
    this.form.addEventListener('submit', e => {
	    e.preventDefault()
	    self.fields.forEach(field => {
        const input = document.querySelector(`#${field}`)
        self.validateFields(input)
      })
    })
  }
  
  validateOnEntry() {
    let self = this
    this.fields.forEach(field => {
      const input = document.querySelector(`#${field}`)
      
      input.addEventListener('input', event => {
        self.validateFields(input)
      })
    })
  }
  
  validateFields(field) {
  
    // Check presence of values
    if (field.value.trim() === "") {
      this.setStatus(field, `${field.previousElementSibling.innerText} cannot be blank`, "error")
    } else {
      this.setStatus(field, null, "success")
    }
    
    // check for a valid email address
    if (field.type === "email") {
      const re = /\S+@\S+\.\S+/
      if (re.test(field.value)) {
        this.setStatus(field, null, "success")
      } else {
        this.setStatus(field, "Please enter valid email address", "error")
      }
    }
    
    // Password confirmation edge case
    if (field.id === "password_confirmation") { 
      const passwordField = this.form.querySelector('#password')
    
      if (field.value.trim() == "") {
        this.setStatus(field, "Password confirmation required", "error")
      } else if (field.value != passwordField.value)  {
        this.setStatus(field, "Password does not match", "error")
      } else {
        this.setStatus(field, null, "success")
      }
    }
  }

  setStatus(field, message, status) {
    const successIcon = field.parentElement.querySelector('.icon-success')
    const errorIcon = field.parentElement.querySelector('.icon-error')
    const errorMessage = field.parentElement.querySelector('.error-message')

    if (status === "success") {
      if (errorIcon) { errorIcon.classList.add('hidden') }
      if (errorMessage) { errorMessage.innerText = "" }
      successIcon.classList.remove('hidden')
      field.classList.remove('input-error')
    } 
    
    if (status === "error") {
      if (successIcon) { successIcon.classList.add('hidden') }
      field.parentElement.querySelector('.error-message').innerText = message
      errorIcon.classList.remove('hidden')
      field.classList.add('input-error')
    }    
  }
}

const form = document.querySelector('.form')
const fields = ["username", "email", "password", "password_confirmation"]

const validator = new FormValidator(form, fields)
validator.initialize()
!
999px

Console