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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>test</title>
<style>
.rojo{ background-color:red}
.azul{ background-color:blue}
input{font-family:Verdana, Arial, Helvetica, sans-serif; font-size:9px; color:#FFF }
</style>
<script>
/*----lib----*/
function $(id){return document.getElementById(id);}
function addEvent(obj, evType, fn, useCapture){
if (obj.addEventListener){
obj.addEventListener(evType, fn, useCapture);
} else if (obj.attachEvent){
var f=function(){
fn.call(obj,window.event);
}
obj.attachEvent("on"+evType, f);
} else {
obj['on'+evType]=fn;
}
}
/*----lib----*/
function rojo(){
if(window.ultimo){
window.ultimo.className='rojo';
}
this.className='rojo';
window.ultimo=null;
}
function azul(){
this.className='azul';
var el=document.getElementsByTagName('input');
for(var i=0,l=el.length;i<l;i++){
if(el[i]== window.ultimo)
continue;
el[i].className='azul';
}
window.ultimo=this;
}
function asignar(){
var el=document.getElementsByTagName('input');
for(var i=0,l=el.length;i<l;i++){
addEvent(el[i], 'focus', rojo, false);
addEvent(el[i], 'blur', azul, false);
}
}
window.onload=asignar;
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="">
<input name="textfield" type="text" class="azul" />
<input name="textfield2" type="text" class="azul" />
<input name="textfield3" type="text" class="azul" />
</form>
</body>
</html>