for #main-content

I don't want to give any fix height because content can be long and short but if content is short then it should take minimum height 500px.

i need compatibility in all browser.

Is thery any w3c valid and cross browser way without using !important because i read !important should not be used

In conclusion, don’t use the !important declaration unless you’ve tried everything else first, and keep in mind any drawbacks. If you do use it, it would probably make sense, if possible, to put a comment in your CSS next to any styles that are being overridden, to ensure better code maintainability.

I tried to cover everything significant in relation to use of the !important declaration, so please offer comments if you think there’s anything I’ve missed, or if I’ve misstated anything, and I’ll be happy to make any needed corrections.

http://www.impressivewebs.com/everything-you-need-to-know-about-the-important-css-declaration/