HTML5 Form validation. Based on https://jsfiddle.net/Loilo/jdct84x6/3/

index.html

<form>
  <input
    type="number"
    placeholder="Enter 42"
    min="42"
    max="42"
    required>

  <input
    type="email"
    placeholder="Enter your email"
    required>
    
  <button type="submit">Register</button>
</form>

index.js

let submitted = false

// 1. Disable native validation UI with `noValidate`
// 2. On submit, run evaluation and prevent if necessary
const form = document.querySelector('form')
form.noValidate = true
form.onsubmit = evt => {
  submitted = true
  setTimeout(() => {
    submitted = false
  }, 0)

	if (!form.checkValidity()) {
  	evt.preventDefault()
  }
}

// Iterate over fields in form
let invalidOnSubmit = false
for (const field of form.querySelectorAll('input, textarea, select')) {

	// Add error container
	field.insertAdjacentHTML('afterend', '<div class="error"></div>')
  
  // Show message on `invalid` event
	field.oninvalid = () => {
    if (submitted && !invalidOnSubmit) {
      invalidOnSubmit = true
      setTimeout(() => {
        invalidOnSubmit = false
      }, 1000)
      
      field.focus()
    }

  	field.classList.add('invalid')
    field.nextSibling.textContent = field.validationMessage
    
    // Reset invalid state & error message on `input` event, trigger validation check
    const inputHandler = () => {
	    field.oninput = null
      field.nextSibling.textContent = ''
      field.classList.remove('invalid')
      field.checkValidity()
    }
    field.oninput = inputHandler
  }
}

style.css

/* Global styles */

html {
  height: 100%;
}

body {
  font-family: sans-serif;
  font-weight: 300;
  font-size: 20px;
  background: linear-gradient(to top, #e0e7ef, #eef2f7);
  min-height: 100%;
  color: #798594;
  line-height: 1.6;
}

/* Form styles */

form {
  background: #f8fafb;
  padding: 3em 1em;
  border-radius: 3px;
  box-shadow: 0 0.1em 0.4em #b7ccde;
  max-width: 15em;
  margin: 2em auto;
}

button, input, textarea, select {
  font: inherit;
  color: inherit;
  padding: 0.4em 0.6em;
  border: 1px solid #ccc;
  border-radius: 4px;
  width: 100%;
  box-sizing: border-box;
  margin-bottom: 0.5em;
}

button {
  cursor: pointer;
  background-color: #e9f1ff;
  color: #7e93b7;
}

input {
  display: block;
  position: relative;
  z-index: 1;
}

input::placeholder {
  color: #bbb;
}

/* Error styles */

.invalid {
  box-shadow: 0 0.15em 0.15em -0.1em rgba(4, 4, 4, 0.1);
}

.error {
  display: none;
  margin-bottom: 0.5rem;
  margin-top: calc(-0.5rem - 4px);
  margin-left: 2px;
  margin-right: 2px;
  padding: calc(0.5rem + 4px) 0.6rem;
  background-color: #ffefeb;
  color: #b75a41;
  border: 1px solid #e6917a;
  border-radius: 0 0 4px 4px;
  animation: show-error 0.3s;
  font-size: 0.8em;
}

.invalid + .error {
  display: block;
}

@keyframes show-error {
  from {
    transform: translateY(-2em) scaleY(0.2);
  }
  
  to {
    transform: none;
  }
}