validar con css
.formulario input:required:valid, .contact_form textarea:required:valid {
box-shadow: 0 0 5px #5cd053;
border-color: #28921f;
}
.formulario input:required:invalid, .contact_form textarea:required:invalid {
box-shadow: 0 0 5px #d45252;
border-color: #b03535;
}