Hello,

I'm trying to make a textarea of user's comments, in which they write what they have to say, then click submit, and it permanently changes the page's HTML, within the textarea to what they put in. Is there a way to do this?

For example, there's a blank textarea. Somebody types something into that textarea, and hits Submit, now the page updates, changing the textarea's value to what they put in, visible to all. It literally changes the page's HTML, permaneltly, but in a given element.

It's kind of like a controlled hack, but it's the only way I know of to get a commenting system in with JavaScript.

Javascript is purely a client side operation, so cannot "change" the page for anyone other than the user/viewer where the page is being displayed.
So without some server side code and a database to collect and store the comments it can't be done simply.
The option without server side code would be to utilise a third party server/database to receive and store the comments and collect and display them using "AJAX" methods or forms in iframes