What went wrong with the nth-child () selector?

I don"t know how to describe the problem either.
Let"s code first
this is normal. 

*{
            margin: 0;
            padding: 0;
        }
        div{
            margin: 0 auto;
            width: 600px;
            height: 400px;
            background: -sharpabcdef;
        }
        div > .oUl >li{
            list-style: none;
            width:150px;
            height:400px;
            background: url(../images/5.jpg);
            background-repeat: no-repeat;
            float: left;
        }
        .oUl li:nth-child(1){
            background-position: 0 0 ;
        }
        .oUl li:nth-child(2){
            background-position: -150px 0 ;
        }
        .oUl li:nth-child(3){
            background-position: -300px 0 ;
        }
        .oUl li:nth-child(4){
            background-position: -450px 0 ;
        }
  
<div>
        <ul class="oUl">
            <li>1</li>
            <li>2</li>
            <li>3</li>
            <li>4</li>
        </ul>
    </div>

li:nth-child .oUl

limarginmargin

what is the principle of this? I am confused. I have come to ask the bosses for advice.

Css css3
Apr.01,2021

this is the priority issue of CSS.
first of all, you need to understand that the background of CSS is actually an omitted way of writing. See

Previous: For the li tab of dynamic rendering in react, click to switch styles.

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MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3b1bc-2c296.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3b1bc-2c296.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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