HTML:
<label id="input-label" class="invalid">
ZIP code: <input id="zipcode" />
<div class="valid-message">
Valid
</div>
<div class="invalid-message">
Invalid
</div>
</label>
CSS:
#input-label.valid .valid-message { display: block; }
#input-label.valid .invalid-message { display: none; }
#input-label.invalid .valid-message { display: none; }
#input-label.invalid .invalid-message { display: block; }
Javascript
function isValidZip(z) {
return ['12345','67890'].indexOf(z) != -1;
}
var label = document.getElementById('input-label');
var input = document.getElementById('zipcode');
input.onkeydown = function() {
label.className = isValidZip(input.value) ? "valid" : "invalid";
}