:focus-within伪类选择器的3种应用

<form><input type="text"></form>

form {
    transition: outline .2s;
    outline: 2000px solid hsla(0,0%,100%,0);
    position: relative;
    z-index: 1;
}
form:focus-within {
    outline: 2000px solid hsla(0,0%,100%,1);
}

<div class="textarea-x">
   <textarea id="forFocus" maxlength="250" rows="5"></textarea>
   <label for="forFocus" class="textarea-count">0/250</label>
</div>

.textarea-x {
    max-width: 298px;
    border: 1px solid #d0d0d5;
    border-radius: 4px;
    background-color: #fff;
    padding-bottom: 24px;
    transition: border-color .2s;
    overflow: hidden;
    position: relative;
}
.textarea-count {
    position: absolute;
    font-size: 12px;
    line-height: 16px;
    bottom: 5px; right: 10px;
    color: #999;
}
.textarea-x > textarea {
    display: block;
    width: 100%;
    padding: 7px 9px;
    border: 0;
    background: none;
    -ms-box-sizing: border-box;
    box-sizing: border-box;
    outline: 0;
    resize: none;
}
.textarea-x:focus-within {
    border-color: #00a5e0;
}

<div class="details">
    <a href="javascript:">我的消息</a>
    <div class="box">
        <li>1</li>
        <li>2</li>
        <li>3</li>
    </div>
</div>

.box {
    display: none;
}
.details:focus-within .box {
    display: block;
}