SOURCE

function sample1_func() {
    var text_area = document.getElementById("sample1_desc");
    var a, b;
    while (isNaN(a)) a = Number(prompt("请输入第一个数字"));
    while (isNaN(b)) b = Number(prompt("请输入第二个数字"));
    text_area.innerHTML += a + "+" + b + "=" + (a + b) + "<br>";
}
var obj = document.getElementById("sample1");
obj.onclick = sample1_func;

<p>HTML <b>内容</b>展示：<span>请点击我...</span></p>
<p>CSS <b>样式</b>展示：<span class="sample1">请点击我...</span></p>
<p>JavaScript <b>行为</b>展示：<span class="sample1" id="sample1">请点击我...</span></p>
<p id="sample1_desc"></p>

.sample1 {
    color: #339999;
    text-decoration: underline;
    cursor:pointer;
    border: 3px solid red;
}

#sample1_desc {
    width: 40%;
    height: 100px;
    border: 5px solid #000;
    border-radius: 20px;
    overflow: auto;
    text-indent: 0;
    padding-left: 2em;
}