<!--[if gte IE 7]>  //gte means >= IE7
    <link rel="stylesheet" type="text/css" href="/stylesheets/ie.css" />
    <p>Both Internet Explorer 8 and Internet Explorer 7 will receive this style sheet. </p>
    <![endif]-->

    <!--[if IE 7]> //means = IE 7
    <p>You're using a recent version of Internet Explorer 7.</p>
    <![endif]-->

    <!--[if gt IE 6]> //means > IE 6
    <p>You're using a recent version of Internet Explorer.</p>
    <![endif]-->

    <!--[if lt IE 6]> // meas < IE 6
    <p>Hm. You should upgrade your copy of Internet Explorer.</p>
    <![endif]-->

    <![if !IE]> means is not IE
    <p>You're not using Internet Explorer.</p>
    <![endif]>

注：

User-Agent String and Version Vector white paper:http://code.msdn.microsoft.com/Release/ProjectReleases.aspx?ProjectName=ie8whitepapers&ReleaseId=531

Understanding User-Agent Strings:http://msdn.microsoft.com/zh-cn/library/ms537503(en-us,VS.85).aspx

How do I keep my site and add-ons working with Internet Explorer 8:http://www.microsoft.com/windows/products/winfamily/ie/ie8/readiness/DevelopersExisting.htm