Note: You will need to delete README.md file after you customize your Proxy site. Else url code inside Cloudflare workers will fetch this file to show you in your browser.
This https://github.com/51sec/jsproxy folder is forked from yangmyc/jsproxy 's repository. The main purpose to create this repository is to customize the original html index_v3.html file and config.js file, so you will be able to add some of more your own stuff in it.
More detail: https://blog.51sec.org/2020/03/build-reverse-proxy-using-cloudflare.html
Demo url: https://proxy.itprosec.com/