.responsive-background{
    width: 100%;
    background-image: url('./img/dall.jpg'); 
    background-repeat: no-repeat;
    background-size: cover;
    background-position: center;
    aspect-ratio: 4 / 3; /* アスペクト比を保持してレスポンシブ */
}

body {
   margin: 0;
   padding: 0;
   background:red;
}


/* https://dominantmotion.com/design/html-css/background-responsive/ */