A happenstance use of https://addons.mozilla.org/en-US/firefox/addon/save-back-to-file-from-dom/.

Store this page on (save to) your local file system (disk) and open from there to try out.

Click text in below white <div, edit, then save back to file.

# some preformatted text

<p>Hello world!</p>

console.log("Hello world!");

Editing features you are experiencing on this page are courtesy of your browser, contenteditable="true".