代码：
<!doctype html>
<html>
<head>
</head>
<body>
<input id="input" type="text" value="请输入关键词">
</body>
<script>
window.onload = function() {
var defaultValue = "请输入关键词";
var input = document.getElementById("input");
input.style.color = "grey";
input.onfocus = function() {
if (this.value == defaultValue) {
setCursorPosition(this, 0);
}
};
input.onkeypress = function(e) {
e = e || window.event;
var key = e.charCode || e.keyCode || e.which;
if (this.value == defaultValue) {
this.value = "";
this.style.color = "black";
}
if (this.value.length == 1 && key == 8) {
this.value = defaultValue;
this.style.color = "grey";
setCursorPosition(this, 0);
}
};
};
function setCursorPosition(elem, index) {
if (elem.setSelectionRange) {
elem.focus();
elem.setSelectionRange(index, index);
}
else if (elem.createTextRange) {
var range = elem.createTextRange();
range.collapse(true);
range.moveEnd('character', index);
range.moveStart('character', index);
range.select();
}
}
</script>
</html>