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CSS

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HTML 

              
                <form class="form" action="./" method="POST">
  <div class="form__title">Sign up</div>
  <p class="form__desc">
    Lorem ipsum dolor sit amet consectetur adipisicing elit. Eos, nostrum? Laudantium molestiae explicabo numquam animi corrupti debitis ratione, accusamus nemo optio eum vero nesciunt facere aperiam! Labore pariatur accusantium exercitationem.
  </p>
  <div class="form__item">
    <label for="givennames" class="form__label">Given Names</label>
    <input type="text" class="form__input" name="givennames" id="givennames" placeholder="Enter your given name(s)">
    <span class="form__error">A sample error message</span>
  </div>
  <div class="form__item">
    <label for="numusers" class="form__label">Number of Users</label>
    <input type="number" class="form__input form__input--small" name="numusers" id="numusers" min="1" max="5" placeholder="How many users? (Max. 5)">
    <span class="form__error">A sample error message</span>
  </div>
  <div class="form__item">
    <label for="expirydate" class="form__label">Expiration Date</label>
    <input type="date" class="form__input form__input--small" name="expirydate" id="expirydate" placeholder="Enter an expiration date">
    <span class="form__error">A sample error message</span>
  </div>
  <div class="form__item">
    <label for="message" class="form__label">Message</label>
    <textarea maxlength="500" class="form__input" name="message" id="message" placeholder="Enter a detailed message about why you wish to signup. (Max. 500 chars)"></textarea>
    <span class="form__error">A sample error message</span>
  </div>
  <div class="form__item">
    <button class="form__btn" type="submit">Sign Up</button>
  </div>
</form>
!

CSS 

              
                @import url("https://fonts.googleapis.com/css?family=Source+Sans+Pro:400,400i,600,600i&display=swap");

.form {
  font-family: "Source Sans Pro", sans-serif;
  font-size: 16px;
}

.form * {
  box-sizing: border-box;
  line-height: 1.5;
}

.form__title {
  font-size: 2em;
  font-weight: 600;
}

.form__item {
  display: flex;
  flex-direction: column;
}

.form__item > * {
  align-self: flex-start;
}

.form__label {
  font-weight: 600;
  padding: 10px 0;
}

.form__input {
  -webkit-appearance: none;

  width: 100%;
  max-width: 425px;

  /* Fix for Safari/iOS date fields */
  min-height: calc(0.9em + (0.8em * 2) + 0.6em);

  padding: 0.8em;
  font-size: 0.9em;
  font-family: "Source Sans Pro", sans-serif;

  outline: none;
  border: 1px solid #dddddd;
  border-radius: 4px;
  background: #f9f9f9;
  transition: background 0.25s, border-color 0.25s, color 0.25s;
}

.form__input:focus {
  background: #ffffff;
}

.form__input::placeholder {
  color: #bbbbbb;
}

.form__input--error {
  color: #d50000;
  background: #fff8f8;
  border-color: #d50000;
}

.form__input--error::placeholder {
  color: #ffbfbf;
}

.form__error {
  padding-top: 10px;
  color: #d50000;
  font-size: 0.9em;
  visibility: hidden;
}

.form__input--error + .form__error {
  visibility: visible;
}

.form__input--small {
  max-width: 250px;
}

textarea.form__input {
  resize: none;
  min-height: 200px;
}

.form__btn {
  font-family: "Source Sans Pro", sans-serif;
  font-weight: 600;
  font-size: 1.1em;
  padding: 10px 16px;
  margin: 10px 0;

  color: #ffffff;
  background: #14b64a;
  border: 2px solid #0fa942;
  border-radius: 5px;

  cursor: pointer;
  outline: none;
}

.form__btn:active {
  background: #0fa942;
}
!

JS 

              
                document.addEventListener("DOMContentLoaded", function() {
  document.querySelectorAll(".form__input").forEach(function(input) {
    input.addEventListener("input", function() {
      input.className = input.className.replace(/form__input--error ?/, "");
    });
  });
});
!
999px

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