:optional CSS pseudo-class

label {
  display: block;
  margin-top: 1em;
}

.req {
  color: red;
}

*:optional {
  background-color: palegreen;
}

<form>
  <label for="name">Name: <span class="req">*</span></label>
  <input id="name" name="name" type="text" required="">

  <label for="birth">Date of Birth:</label>
  <input id="birth" name="birth" type="date">

  <label for="origin">How did you find out about us? <span class="req">*</span></label>
  <select id="origin" name="origin" required="">
    <option>Google</option>
    <option>Facebook</option>
    <option>Advertisement</option>
  </select>
  <p><span class="req">*</span> - Required field</p>
</form>

:optional {
  /* ... */
}

<form>
  <div class="field">
    <label for="url_input">Enter a URL:</label>
    <input type="url" id="url_input" />
  </div>

  <div class="field">
    <label for="email_input">Enter an email address:</label>
    <input type="email" id="email_input" required />
  </div>
</form>

label {
  display: block;
  margin: 1px;
  padding: 1px;
}

.field {
  margin: 1px;
  padding: 1px;
}

input:optional {
  border-color: rebeccapurple;
  border-width: 3px;
}