css parent-child fill

<html>
    <head>
        <meta http-equiv="content-type" content="text/html; charset=utf-8">
        <title>GUI_container</title>
        <style type="text/css">
div{-moz-box-sizing:border-box;}

.GUI_window
{
    border: #777777 4px solid;
    -moz-border-image: url(GUI_window.png) 4 repeat;
}
.GUI_window_title
{
    border: #777777 0px solid;
    border-left-width:5px;
    border-right-width: 18px;
    -moz-border-image: url(GUI_title.png) 0 18 0 5  repeat;
    margin-left:10px;
}
.GUI_window_title_buttons
{
    float: right;
    width:10px;
    height:10px;
    margin-left: 3px;
}

.GUI_window_content
{
    border: #777777 4px solid;
    -moz-border-image: url(GUI_content.png) 4 repeat;
}
.GUI_window_resizer
{
    position: relative;
    left: 4px;
    top: -5px;
    float: right;
    width: 9px;
    height: 9px;
    background: #f00;
}

#testwin
{
    width: 400px;
    height: 300px;
}
        </style>
    </head>
    <body>
        <div id="testWin" class="GUI_window">
            <span class="GUI_window_title">title</span>
            <div class="GUI_window_title_buttons"></div>
            <div class="GUI_window_content">
                content<br>content
            </div>
            <div class="GUI_window_resizer"></div>
        </div>

    </body>
</html>