设置文本输入框默认值
有默认值，有一个灰色文字样式（提示用户的信息）；
获得焦点后，清空默认值让用户输入文字，这时失去焦点后，给一个样式名来改变文字颜色，这种情况下再次获得焦点时不清空用户已输入的值。
如果用户没有输入任何值，返回默认值，灰色文本样式
<!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>
<title>DEMO</title>
<style type="text/css">
#form div {
position: relative;
}
#form .tip {
position: absolute;
top: 1px;
left: 3px;
color: #ccc;
}
#form input {
color: red;
}
</style>
</head>
<body>
<div id="form">
<div><label class="tip" for="textbox1">LABEL1</label><input id="textbox1" type="text" /></div>
<div><label class="tip" for="textbox2">LABEL2</label><input id="textbox2" type="text" /></div>
<div><label class="tip" for="textbox3">LABEL3</label><input id="textbox3" type="text" /></div>
<div><label class="tip" for="textbox4">LABEL4</label><input id="textbox4" type="text" /></div>
<div><label class="tip" for="textbox5">LABEL5</label><input id="textbox5" type="text" /></div>
<div><label class="tip" for="textbox6">LABEL6</label><input id="textbox6" type="text" /></div>
</div>
<script type="text/javascript">
var form1 = document.getElementById("form"),
isIE = /*@cc_on!@*/0;
function focusin(event) {
var e = isIE? window.event: event,
target = isIE? e.srcElement: e.target;
target.previousSibling.style.display = "none";
target.parentNode.className += " focus";
}
function focusout(event) {
var e = isIE? window.event: event,
target = isIE? e.srcElement: e.target;
if(target.value === "") {
target.previousSibling.style.display = "inline";
}
target.parentNode.className = target.parentNode.className.replace(/\s+focus/, "");
}
if(isIE) {
form1.onfocusin = focusin;
form1.onfocusout = focusout;
} else {
form1.addEventListener("focus", focusin, true);
form1.addEventListener("blur", focusout, true);
}
</script>
</body>
</html>
