:root{ --bg-overlay: rgba(0,0,0,0.35); --bg-blur: 0px; }
html, body { min-height: 100%; }
body::before{
  content: ""; position: fixed; inset: 0; z-index: -1;
  background:
    linear-gradient(var(--bg-overlay), var(--bg-overlay)),
    image-set( url("/assets/images/background.avif") type("image/avif") 1x );
  background-size: cover; background-repeat: no-repeat; background-position: center; background-attachment: fixed;
  filter: blur(var(--bg-blur));
}
@media (max-width: 900px){ body::before{ background-attachment: scroll; } }