I think everyone of us has faced the situation when he found that some website is missing something which could have been done by just a bit of JavaScript. But, unfortunately we can do nothing regarding this except contacting the website author to try to convince him that this change is really good and he should do it.
I can now tell you that you don't have to go through all this hustle because you now have GreaseMonkey which provides you with the ability to run JavaScript across certain websites. It is like injecting the JavaScript you need into a website but for sure at your side only, not in the source website itself.
Using GreaseMonkey you can perform so many cool things. If you want to know more about GreaseMonkey, you can check this article