<!DOCTYPE html>
<html>
<style>
  body {
    height: 4000px;
    margin: 0;
    overflow: hidden;
  }
  #clip {
    position: absolute;
    top: 450px;
    left: 300px;
    background: red;
    width: 400px;
    height: 400px;
    clip-path: inset(0);
  }
  #fixed {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: linear-gradient(green, blue);
  }
</style>
<div id="clip">
  <div id="fixed"></div>
</div>