Using just semantic CSS Pseudo-Classes you can help define important states for form elements that ensure the user provides the correct data without frustration.

input:focus {
  outline: none;
  box-shadow: 3px 3px 1px rgba(0,0,0,0.2);
  border: 1px solid rgba(0,0,0,0.5);
}
fieldset:disabled 
 
{ /* :enabled opposite */
  opacity: 0.5;
}
input:checked + label {
  font-style: italic;
}
input:invalid {
  border-color: red;
}
input:valid {
  border-color: green;
}
input:required {
  border-width: 2px;
}