Why the same program will produce different output style in browser?

dear teacher,
From the begining i was runing code in internet explorer only, but i now instaled firefox, its showing output in fully diferent format. specialy in CSS Programs. Why it so? how we have to write the code , which should display same in all browser, i tried it, but i did't get..

Thank you teacher.......:)

hi Uma,

you are correct, some time you will get this issue in case of mozila and IE.
that is reason, we had 2nd chapter for getting user is using which browser, and accordingly we do coding , In case of CSS you can write 2 css 1 for IE and one for mozila.
like below example.

<!--[if IE 6]>
<link href="qaforie.css" rel="stylesheet" type="text/css" />
<![endif]-->