You can only store those changes to a file on your local pc. Other people are still getting the html from the webserver and you do not have access to overwrite that(as seen on basic 4 and 5). You could use a plugin like greasemonkey to always change a specific web page in a specific way in your browser, but even then it'll only be changed for you.
Now if you could find a real exploit on that server, maybe you could edit files on the server and change the actual file being sent to everyone. But this is usually a lot harder then just editing the html.
greets and Happy hacking
<Yoda> if someone says something i don't like, i ban him, ban whoever defends him, and then ban the witnesses...