Is this a right code?

I know I cant put (alert box) javascript codes in css, but I've read something and says that there is a way to put javascript code by putting it in the html box, I've got this code from someone, I just viewed her source code and found this:
<img id="mycode" style="height:expr/**/ession(alert('her message'))(alert('her message')); width:expr/**/ession(document.all.mycode.style.height='0px')" />

i've tried it adding the code <object></object> and it work but when i've edited the messages, it doesnt work anymore.

is this the right code for using this javascript code and what is the use of this code? :img id="mycode" style="height:expr/**/ession...width:expr/**/ession(document.all.mycode.style.height='0px')" />

and if there are other codes for me to be able to use javascript codes?

P.S.
sorry for my bad english.