Example code sharing of css pseudo-class combination usage

Using pseudo-class combination, you can use css instead of js to achieve the purpose, less js

.textarea:empty:before {
    display: block;
    content: '请输入';
    color: #ababab;
    font-size: 15px;
    text-align: left;
}<br>//表示：当textarea元素为空时即没有输入时，有“请输入”这句提示语显示
  
.textarea:focus:before {
    display: none;
}<br>//当textarea元素聚焦时，刚刚用before伪类加的“请输入”就会隐藏<br><br>.textarea:empty:after {
content: &#39;请认真填写。&#39;;
   color: #ababab;
   font-size: 11px;
   text-align: left;
}
.textarea:focus:focus {
    display: none;
}
first-child:after
.order:empty+.a {
    display: block;
}<br>//order元素为空时，它下面的那个元素a显示

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