:valid

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

input:valid {
  background-color: ivory;
  border: none;
  outline: 2px solid deepskyblue;
  border-radius: 5px;
  accent-color: gold;
}

<form>
  <label for="email">Email Address:</label>
  <input id="email" name="email" type="email" value="na@me@example.com" />

  <label for="secret">Secret Code: (lower case letters)</label>
  <input id="secret" name="secret" type="text" value="test" pattern="[a-z]+" />

  <label for="age">Your age: (18+)</label>
  <input id="age" name="age" type="number" value="5" min="18" />

  <label
    ><input name="tos" type="checkbox" required checked /> - Do you agree to
    ToS?</label
  >
</form>

:valid {
  /* ... */
}

<div>
  <label for="fname">First name *: </label>
  <input id="fname" name="fname" type="text" required />
  <span></span>
</div>

input + span {
  position: relative;
}

input + span::before {
  position: absolute;
  right: -20px;
  top: 5px;
}

input:invalid {
  border: 2px solid red;
}

input:invalid + span::before {
  content: "✖";
  color: red;
}

input:valid + span::before {
  content: "✓";
  color: green;
}