So, this is just a normal HTML page with the <html>/<head>/<body> tags omitted. Browsers don't care if those are missing. In fact, you can save this text to a .html file and open it in Chrome and it will work the same as the "magic" URL.
The page creates a <div> and uses CSS to set its font size and make it fill the window. Then it loads the Ace Editor library [1] and initializes an editor instance in that <div>. Finally, it selects the Monokai theme and JavaScript syntax highlighting.
(I omitted the type="text/css" and type="text/javascript" because they aren't needed.)
Or, you know, you could just use Notepad as Notepad. Or a real text editor, or something like Simplenote.
Snark aside, this would be a more interesting post if the author gave some reason why this was a good idea. As far as I can tell, it's strictly worse than a text editor. I mean, it is an interesting hack, but the post doesn't frame it at all.
My feelings exactly: C-r n gets me straight to taking a note in Emacs (via org-mode), complete with link to what I was in when I took the note, timestamp, time tracking and all the other org-mode/Emacs goodness (tags, tables, links, etc). Maybe if you live in your browser the OP makes sense, but for those of us who live in Emacs (mainly for code), it makes more sense to use a real editor.
Org-mode looks great, new to me. I made my own much more primitive thing, I've been slowly evolving over a decade and a half, so yes, this is a knock on the head. Old outline mode never took with me.
Ever since I first saw the aloha text editor I realised content-editable is going to have a massive impact - this was one that is totally obvious in hindsight but has completely floored me.
And it works in my iphone. Cannot actually save but still its nice to know.