It sure would be a shame if say, if you clicked a link, the page that loaded could just talk to the whole internet from your computer, right? Imagine the malice.
So browsers stop that shit cold. Nobody would use 'em if every time you got on the web it stole your TV.
- Submit forms
- Load scripts (and therefore make a GET request) from any site.
- Make any requests back to the site it came from.