This is sort of hack which helps you write CSS specific to IE6 / IE7.

EX:

_display:none;/* IE 6 Fix */
*display:none;/* IE 7 Fix */

NOTE: This code is not recommended if you are writing code that has to comply to CSS standards. In such cases you may have to switch to alternate methods.

One of the best methods is to use conditional CSS Comments as follows:

        <!--[if IE]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->

        <!--[if IE 6]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if IE 7]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if IE 8]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if IE 9]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if gte IE 8]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if lt IE 9]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if lte IE 7]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if gt IE 6]>
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <![endif]-->
        <!--[if !IE]> -->
                <link rel="stylesheet" type="text/css" href="ie.css" media="all" />
                <script type="text/javascript" src="ie.js"></script>
        <!-- <![endif]-->