css height：0到height：auto 的动画

问题？容器height：0到height：auto没有动画效果

#child0 {
    height: 0;
    overflow: hidden;
    background-color: #dedede;
    -moz-transition: height 1s ease;
    -webkit-transition: height 1s ease;
    -o-transition: height 1s ease;
    transition: height 1s ease;
}
#parent0:hover #child0 {
    height: auto;
}
#child40 {
    height: 40px;
    overflow: hidden;
    background-color: #dedede;
    -moz-transition: height 1s ease;
    -webkit-transition: height 1s ease;
    -o-transition: height 1s ease;
    transition: height 1s ease;
}
#parent40:hover #child40 {
    height: auto;
}
h1 {
    font-weight: bold;
}

<hr />
<div id="parent0">
    <h1>Hover me (height: 0)</h1>
    <div id="child0">Some content
        <br />Some content
        <br />Some content
        <br />Some content
        <br />Some content
        <br />Some content
        <br />
    </div>
</div>
<hr />
<div id="parent40">
    <h1>Hover me (height: 40)</h1>
    <div id="child40">Some content
        <br />Some content
        <br />Some content
        <br />Some content
        <br />Some content
        <br />Some content
        <br />
    </div>
</div>

解决办法（用max-height去代替height）

#menu #list {
    max-height: 0;
    transition: max-height 0.15s ease-out;
    overflow: hidden;
    background: #d5d5d5;
}

#menu:hover #list {
    max-height: 500px;
    transition: max-height 0.25s ease-in;
}

<div id="menu">
    <a>hover me</a>
    <ul id="list">
        <!-- Create a bunch, or not a bunch, of li's to see the timing. -->
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
        <li>item</li>
    </ul>
</div>

原文章为https://stackoverflow.com/questions/3508605/how-can-i-transition-height-0-to-height-auto-using-css