Creating your own online code editor - where to start?

Question

JavaScript

I want to create my own online code editor, first HTML and CSS, then I will develop it.
It is necessary that in one window on the page the user can write html with inline styles, in the other - css, as if it were a separate connected file, and in the third, without refreshing the page, the result is shown.
Basic web development skills are available, but I don’t really know where to start, what to use

Answer 1

It's not hard to get started. A couple of textarea, a block to display and an update on the input event.
jsfiddle.net/Stalk/tfhvgg6z/show/light
There are little things left - to fasten syntax highlighting, for example.
But if you take it ready , then everything is easier.

Answer 2

You need to start with a parser (well, or ready to take it), so that it can tokenize elements, this is the most difficult thing. The rest is already trivial.

Answer 3

Ace