Textarea that can do syntax highlighting on the fly?

It's not possible to achieve the required level of control over presentation in a regular textarea.

If you're OK with that, try CodeMirror or Ace or Monaco (used in MS VSCode).

From the duplicate thread - an obligatory wikipedia link: Comparison of JavaScript-based source code editors

Here is the response I've done to a similar question (Online Code Editor) on programmers:

First, you can take a look to this article:
Wikipedia ― Comparison of JavaScript-based source code editors.

For more, here is some tools that seem to fit with your request:

• EditArea ― Demo as FileEditor who is a Yii Extension ― (Apache Software License, BSD, LGPL)

  Here is EditArea, a free javascript editor for source code. It allow to write well formated source code with line numerotation, tab support, search & replace (with regexp) and live syntax highlighting (customizable).

• CodePress ― Demo of Joomla! CodePress Plugin ― (LGPL) ― It doesn't work in Chrome and it looks like development has ceased.

  CodePress is web-based source code editor with syntax highlighting written in JavaScript that colors text in real time while it's being typed in the browser.

• CodeMirror ― One of the many demo ― (MIT-style license + optional commercial support)

  CodeMirror is a JavaScript library that can be used to create a relatively pleasant editor interface for code-like content ― computer programs, HTML markup, and similar. If a mode has been written for the language you are editing, the code will be coloured, and the editor will optionally help you with indentation

• Ace Ajax.org Cloud9 Editor ― Demo ― (Mozilla tri-license (MPL/GPL/LGPL))

  Ace is a standalone code editor written in JavaScript. Our goal is to create a web based code editor that matches and extends the features, usability and performance of existing native editors such as TextMate, Vim or Eclipse. It can be easily embedded in any web page and JavaScript application. Ace is developed as the primary editor for Cloud9 IDE and the successor of the Mozilla Skywriter (Bespin) Project.

CodePress does this, as does EditArea. Both are open source.