centering(margin:0 auto) takes no effect(not working)

#why does action centering(margin:0 auto) takes no effect(not working)? A:there can be mulitple reasons.

1. It may because of browsers' fault.Make sure you are not using old IE or any unsupported browsers.
2. It may because of block-level element or inline-level element related .
3. It may because of you didn't add 'width' property .
   #Ok, I have addressed the reason. Now how to fix it? check out this post for details.