Can I mock network requests from a cross-origin iframe using vanilla JavaScript?

Question

I want to embed a cross-origin iframe (for example: a Google page) in my HTML and intercept or mock some of its network requests. It would be without using browser extensions, proxies, or external tools. It needs to work out of the box when a user visits the page.

<iframe src="https://www.google.com" description="My embedded browser"></iframe>

Is there any way to intercept or stub requests from this iframe using only vanilla JavaScript and HTML? Or is this blocked by browser security policies?

My goal here is mostly to learn. I’m experimenting with how HTTP, certificates, and HTML interact, and I was curious if I could reuse a frontend with a custom backend.

• mplungjan: 1. If you own the page, patch the fetch const origFetch = window.fetch; window.fetch = mockFetch. 2. If you do NOT own the page, you can consider to Your browser will request data from an IP address. If you modify your host file 127.0.0.1 api.example.com, you can mock it all you want. You likely have to issue a certificate so localhost runs https
• Valentin Vignal: 3 downvotes and the question got closed because it is "missing details or clarity". But except for @KIKOSoftware, no one has given any feedback, asked any additional information, or told me what kind of details are missing. I'm not sure what kind of details I need to add and how to edit my question... I updated my question but without more information I'm shooting in the dark here and I need help to know what is missing
• KIKO Software: What if you downloaded the HTML of the page with Javascript, using fetch() for instance? You can then modify the HTML to add some Javascript to monitor request, and then you put that HTML in the iframe. This could work, but might not work if the resources, that the HTML wants to load, are protected against cross-origin requests. Those request are now coming from your domain. In other words, this is difficult. Perhaps people closed your question because you didn't tell them why you want to do this?
• Valentin Vignal: Thanks @KIKOSoftware, I really appreciate your suggestions. From what I understand, there’s no straightforward way to do this purely on the frontend, especially with HTTPS and certificate issues. I was hoping that mocking everything client-side would avoid those problems, but it seems more complex than expected. My goal here is mostly to learn. I’m experimenting with how HTTP, certificates, and HTML interact, and I was curious if I could reuse a frontend with a custom backend.
• KIKO Software: In general people who downvote don't give a reason. It would be kind and helpful, but most just assume you take the effort to read the help section that explains it all. Short questions with little information often get downvoted. Here, for instance, you put your motivation in a comment whereas this should be part of the question. You also didn't tell us all the things you've already tried. Stack Overflow can be rough, but it does rewards good questions as well.