<!DOCTYPE html><html lang="en"><head> <meta charset="UTF-8"> <title>Title</title> <style> .box { width: 200px; height: 200px; background-color: #000; position: relative; } .box:after { content: ''; width: 204px; height: 204px; background-color: red; position: absolute; top: 50%; left: 50%; transform: translate(-50%, -50%); z-index: -1; } </style></head><body><div class="box"></div></body></html>
