<!doctype html>
<html>
    <head>
        <style>
            .contents { border:2px solid skyblue; background-color:#cccccc; margin-block-start:6px; margin-block-end:6px;  }
            .fieldset { position:relative; vertical-align:bottom; inline-size:320px; position:relative; display:inline-block;   border:12px solid navy; padding:28px; }
            .legend {  block-size:64px; inline-size: 284px;background-color:#3080f080; border:8px solid lightblue; font-size:48px; position:absolute;bottom:0;right:0; position:absolute }
            .behind-box { position:absolute;; z-index:-1;  }
           
        </style>
    </head>
    <body>
        <div class=fieldset>
            <div class="behind-box" style="left:0; top:34px; right:0; bottom:0;"></div>
            <div class=legend>Legend Title</div>
            
            <div class=contents>Fieldset Contents</div>
            
            <div class=contents>More Fieldset Contents</div>
            
            
        </div>
    </body>
</html>