What if it's valid? There's a reason we have the phrases "in the red" and "in the black."
Another example where reading the input might be nice:
input[type="cc-number"][value^="4"]+.cc-system-icon {
background-image: url('visa.png');
}input[type="cc-number"][value^="5"]+.cc-system-icon {
background-image: url('master-card.png');
}
