<h1>this is h1</h1>

    <div id="p1">

        <h2>this is h2</h2>

        <input type="text" >

        <div id="c1">1</div>

        <div id="c2">2

            <div id="cc21">21</div>

            <div id="cc22">22</div>

            <div id="cc23">23</div>

        </div>

        <div id="c3">3</div>

        <div id="c4">d4</div>

        <div id="c5">d5</div>

        <div id="c6"> </div><!-- 有一个空格:empty就找不到了 -->

        <div id="c7"></div>

        <div id="c71"><!-- 这种情况:empty也找不到 --></div>

        <div id="c8"><span></span><span>2</span></div>

    </div>