<!doctype html>
<html>
    <head>
        <style>
            fieldset > div:not(.positioned) { border:2px solid skyblue; background-color:#cccccc; margin-block-start:6px; margin-block-end:6px  }
            fieldset { position:relative; margin:0; vertical-align:bottom; inline-size:320px; display:inline-block; border:12px solid navy; padding:28px }
            legend { padding:0; block-size:64px; inline-size: 284px; background-color:#3080f0; border:8px solid lightblue; font-size:48px }
            .positioned { position:absolute; top:10px; bottom:10px; right:10px; left:10px; background-color:darkblue; z-index:-1 }
        </style>
    </head>
    <body>
        <fieldset>
            <legend>Legend Title</legend>
            
            <div>Fieldset Contents</div>
            
            <div>More Fieldset Contents</div>
            
            <div class=positioned></div>
        </fieldset>

    </body>
</html>