Following that logic, wouldn't be everyting that eats content to display them malleable? Image-viewers, audio&video-players, office-applications, MS Paint and Windows Notepad? They all have the core-functionality of showing something to the user which is not embedded in their source-code, while this core-functionality is part of it.

If anything, I would go for web pages as a malleable system. Because with userscripts and userstyles you can change them however you want.