WebPageTest's new 'Edit Response HTML' experiment allows you to freely edit the initial HTML of a website and compare the performance of that edited version to the live website.
View here: https://blog.webpagetest.org/posts/edit-response-html/
By the way... I am currently exploring work opportunities for 2024! If you think I might make a good addition to your team, whether full-time or as an independent contractor, please reach out.