#skating_rink{
    position:relative;
    width:100%;
    aspect-ratio:3733.26/1469.07;
    overflow:hidden;
}
#skating_rink .rink-back,
#skating_rink .rink-front,
#skating_rink #marks,
#skating_rink #particles{
    position:absolute;
    inset:0;
    width:100%;
    height:100%;
    display:block;
}
#skating_rink .rink-back{
    z-index:1;
}
#skating_rink #marks{
    z-index:2;
}
#skating_rink #particles{
    z-index:3;
    pointer-events:none;
}
#skating_rink .rink-front{
    z-index:5;
    pointer-events:none;
}
/* mogu は JS が #skating_rink 内に生成（position:absolute;
 z-index:4） */