What are the cons of using a contentEditable div rather than a textarea?

In divs with contenteditable="true" the content can be html formatted, e.g. text with different colors. See also https://stackoverflow.com/a/40898337/11769765.

It's not the same thing. First semantically, the purpose of a textarea is to write/edit plain text whereas with contentEditable you can edit list for instance (see: htmldemo) Second the behavior is also different. For example, in chrome when you test the link below and that you delete all the content you loose the focus (the div element disappear) which is not the expected behavior, or if it is it's idiot.

The Gmail's mail edit box is also a div with contenteditable="true". The major benefit is it has auto-adjust height as user input text/content. Also it supports rich text inside. You can mimic the Textarea by setting a max height if need.

On the other hand if you want auto height in Textarea, you might have to use js to bind some listener to the oninput hook.

It would work fine, but it'd be a little bit more difficult than a form, simply because you're going to have to wire up your own logic to make the button's click event track down the correct div, get its content, and then perform the submission. The advantage of a textarea is that the browser takes care of all that for you.