The HTML and JSON standards don't get close to this by themselves.
I don't think iframes really do it either, but even if they do nobody uses them that way and a replacement seems okay. It's not really a "competing standard" if nobody wanted to use it.
If you look at https://blockprotocol.org/hub and can survive the <5fps (yikes!), you can see that they've demonstrated reimplementations of <p>, <img>, <h1> by piling mountains of Javascript/JSON on what is (hopefully?) just those standard HTML elements under the hood.
If I have to pick between standard slow-moving HTML and wild west HTML-reimplemented-in-React, I will choose the former every time. I will never have to worry about "my <p> has been abandoned by its maintainer and is now malware!".
I don't think iframes really do it either, but even if they do nobody uses them that way and a replacement seems okay. It's not really a "competing standard" if nobody wanted to use it.