How do I make all floating child elements equal to the longest child elements after clearing floats?

problem description

The

structure is div > ul > li*3
the longest li stretches out the parent element after using double pseudo elements to clear the float, as shown in figure

but not pretty. How to make all child elements equal to the longest

the environmental background of the problems and what methods you have tried

div, ul, li do not set height, attempt to set height:100% for ul and li has no effect

related codes

/ / Please paste the code text below (do not replace the code with pictures)

what result do you expect? What is the error message actually seen?

ask if there is any way

Css
Mar.28,2021

first of all, the flex layout is easy to implement.
I think the landlord wants to ask how to implement a lower version of the browser.
you can use padding-bottom:9999px;margin-bottom:-9999px; to achieve the effect of alignment
implement
https://codepen.io/xboxyan/pe.

http://jsbin.com/sapuxon/1/ed.

Pure css implementation, not optimized, to see if it meets your requirements

flex layout? 

<div class="box">
    <div class="child"></div>
    <div class="child"></div>
    <div class="child"></div>
</div>
/*  */
.box {display: flex;flex-flow: row nowrap;align-items: stretch;background-color: lightyellow;}
.child {flex: 0 0 100px;font-size: 14px;line-height: 30px;color: green;padding: 10px;margin: 20px;background-color: greenyellow;}
.child:nth-child(2) {flex: 1 1 auto;}

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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-64f5-1b3aebc-2c239.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3aebc-2c239.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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