Evil example: evil_js.html
Note: make sure that all of your work is saved before you follow the above link. While not all systems or browsers will crash at the above page, many will, so be careful.
Do I think the principle of having an extendible system to enhance the display of information is a bad thing? Not really, so long as (1) it is entirely optional, and (2) the language is not Turing-complete, and (3) the user is allowed to access, modify, and prohibit *any* action.