We can already change the HTML code for the game page, this is just an example of my game page. https://guruhebat.itch.io/muttergeschichte. But not in a very deep HTML and CSS code, cause when I try to write my own style, it seems that the system deletes my style code. Haven't try the javascript though.
Viewing post in Allow Twitter Feed Embeds
Itch doesn't allow for injecting JS scripts into the Description block. It also doesn't play well with some specific HTML blocks, like <div> and <span>. I'd assume it's because they don't want you breaking the uniformity of the game pages, which is completely understandable; It's something that could easily be abused.