Fijate así:
Código PHP:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
<title>Documento sin título</title>
<script type="text/javascript">
function CambiarControlError(id)
{
var did = document.getElementById(id);
with (did.style) {
borderColor = "#D52F2E";
borderWidth = "3px";
backgroundColor = "#F7D5D5";
}
}
function CambiarControlGood(id)
{
var gid = document.getElementById(id);
with (gid.style) {
borderColor = "#DA9001";
borderWidth = "3px";
backgroundColor = "#FFFFFF";
}
}
function ValidarIngresoUsuario()
{
var us = document.getElementById("usuario");
var ok = document.getElementById("okusuario");
if (us.value.length < 4 || us.value.length > 12) {
CambiarControlError("usuario");
ok.innerHTML = "";
return false
}
else {
CambiarControlGood("usuario");
ok.innerHTML = "OK";
with (ok.style) {
color = "#009933";
fontWeigth = "bold";
}
return true;
}
}
</script>
</head>
<body>
<form>
<input id="usuario" type="text" name="textfield" onkeydown="ValidarIngresoUsuario()" onkeyup="ValidarIngresoUsuario()" />
<input id="okusuario" type="text" name="textfield2" />
</form>
</body>
</html>