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You can try something like this to make 'fixed' background to work on iOS in UNA:
.bx-main { background-color: transparent; } html::before { content: ""; display: block; position: fixed; left: 0; top: 0; width: 100%; height: 100%; z-index: -10; background: url(https://us-east-1.linodeobjects.com/una/bx_posts_photos_resized/g/gh/ghm/ghmnxxsz2c7d6tkt3rtfchrreye6finw.png) no-repeat center center; -webkit-background-size: cover; -moz-background-size: cover; -o-background-size: cover; background-size: cover; }