IE7 THEAD Tag Styling

This code works fine in IE8, Firefox, Chrome etc... but ie7 don't recognize this line:

.defaulttable thead{border-right:55px solid #c7c9cf;}

because of that i see no-border in ie7. How can i solve this problem ?

HTML:

<table id="search_table" class="mtm defaulttable">
   <thead>
     <tr class="sortlinks">
        <td>Hello 1</td>
        <td>Hello 2</td>
     </tr>
   </thead>
   <tbody>
       ...........................
</table>

CSS:

.defaulttable {width:100%;border-collapse:collapse;}
.defaulttable thead td{font-weight:bold;background: url('/static/images/grey_fade_back.png') repeat-x 0 -2px;white-space:nowrap;}
.defaulttable开发者_StackOverflow thead{border-right:55px solid #c7c9cf;}
.defaulttable tbody td{border-right:1px solid #c7c9cf;}
.defaulttable td {border-top:1px solid #c7c9cf;border-bottom:1px solid #c7c9cf;border-left:1px solid #c7c9cf;padding:6px 8px 6px;}
.defaulttable .project_name{background-image:url('/static/images/mini/project.png');background-position:7px 7px;background-repeat:no-repeat;padding-left:30px}
.defaulttable .user{background-image:url('/static/images/mini/user.png');background-position:7px 7px;background-repeat:no-repeat;padding-left:30px}
.defaulttable .job_ad_title{background-image:url('/static/images/mini/pin.png');background-position:7px 8px;background-repeat:no-repeat;padding-left:30px}
.defaulttable .project_category div{}       
.defaulttable .odd{background:#EFEFEF url(/static/images/button-overlay.png) repeat-x scroll 0 0}
.defaulttable .hover{background:#FEFFAF url(/static/images/button-overlay.png) repeat-x scroll 0 0}
.defaulttable .selected{background:#99DFA2 url(/static/images/button-overlay.png) repeat-x scroll 0 0}
.defaulttable .sortlinks a{color:#444}
.defaulttable tfoot {}  

You can use conditional comments to specifically target alternate rules to IE 7:

<!--[if IE 7]>

    <style>
    /* alternate style rules for IE 7 */
    table thead tr th { /* style the `th` not the `thead` */ }
    </style>

<![endif]-->

It's worth noting that for table-headings the th element is more appropriate than td (since it is, literally, a 'table heading').