I doubt Hypercard would have ever gotten anything like CSS. It was unlikely enough that CSS ended up anything like CSS!
https://en.wikipedia.org/wiki/WorldWideWeb
Netscape was going to have a built in editor, but they ran out of time, was added in Netscape 3 Gold, and there by default in 4.
(Pre-Edgmium, Edge's Dev Tools even shared a bunch of code with VS Code and briefly explored hosting a full VS Code experience.)
::ye olde Mozilla Composer appears directly::
No you wouldn't, but you can ;) How high is your tolerance today?
I say tolerance because much more recently I had problems trying to make a chart and tripping over colspan and rowspan stuff. I really just need to sketch the contents out on paper, first...
This is a fundamentally different way of how the environment works. You could stop a program and access the development environment because that was part of the runtime.
Examples of this include elisp and emacs, smalltalk and squeak, and to a lesser extent visual studio and C#.
There have been relatively few ecosystems so integrated. Java and Javascript/html/electron apps are notably not like that at all. There is no one "the" environment for coding those, simply several very popular ones.
My own take is that Apple should have made a "Hypercard"-like thing the "top layer" of a new operating system. In other words, the entire desktop/windows/buttons interface to the main OS would be itself scripted in Hypercard.
Smalltalk takes this a step further of course by making the whole operating system a live environment. The very UI vocabulary one uses can also double as "programming"