well these hacks actually really work, i tested it myself. 

For one CSS file: 

  1. /*IE7 HACKS*/
    html > body #ie7 {background-color:#f00;}
  2. /*IE6 HACKS*/
    * html #ie6 {background-color:#0f0;}
  3. /*IE HACKS*/
    #ie6 {background-color:#00f;}
  4. >FF1.0
    #hackme, x:-moz-any-link {styles for Firefox 1.0 here}#hackme, x:-moz-any-link, x:only-child {restore normal styles for newer versions}
  5. >FF2.0
    #hackme, x:-moz-any-link {styles for Firefox 2.0 here}
    #hackme, x:-moz-any-link, x:default {restore styles for Firefox 3.0 and newer}

For seperate Css files:

  1. For IE:
    <!–[if IE]>
    <link rel="stylesheet" href="ie7.css" type="text/css" media="screen" />
    <![endif]-->

     

  2. Non IE:
    <!--[if !IE]>-->
    <link rel="stylesheet" href="default.css" type="text/css" media="screen" />
    <!--<![endif]-->

List from friends:
1. Thomas ( thank  man :P )

getting transparent div backgrounds working in all browsers

* html #overlay{
background-color: #333;
back\ground-color: transparent;
background-image: url(img/blank.gif);
filter: progid: DXImageTransform.Microsoft.AlphaImageLoader(src="img/overlay.png", sizingMethod="scale");

} 

 

 

list of resource:

  1. http://www.puidokas.com/efficient-conditional-comments/
  2. http://pornel.net/firefoxhack
  3. http://www.webdevout.net/css-hacks#unrecommended-body_empty

i will update the list if i found one :) . any help for updating would be nice