<p>
　　<input type="text" value="" name="user_name" id="user_name" />
　　<span style="padding-left:10px;"><input type="button" value="show" onclick="showValue()" /></span>
</p>
<p id="text"></p>

window.onload = function() {
var user_name = document.getElementById("user_name");
user_name.focus();
}

function showValue() {
var user_name = document.getElementById("user_name");
var text = document.getElementById("text");
if(user_name.value == "") {
alert("Please input some words");
user_name.focus();
} else {
text.innerHTML = user_name.value;
text.setAttribute("class", "text");
}
}

//創建div元素
var text = document.createElement("div");
//把user_name.value的值添加到div元素中來
text.appendChild(user_name.value);
//最后將div元素添加到body中來
document.body.appendChild(text);