$minWidthVisibility: (
    xs: 0em,
    sm: 48em, // 768px
    md: 64em, // 1024px
    lg: 75em, // 1200px
);
@function minWidthVisibility($key) {
    @if map-has-key($minWidthVisibility, $key) {
        @return map-get($minWidthVisibility, $key);
    }

    @warn "Unknown `#{$key}` in minWidthVisibility().";
    @return null;
}
$maxWidthVisibility: (
    xs: 47.999em,
    sm: 63.999em,
    md: 74.999em,
    lg: none,
);
@function maxWidthVisibility($key) {
    @if map-has-key($maxWidthVisibility, $key) {
        @return map-get($maxWidthVisibility, $key);
    }

    @warn "Unknown `#{$key}` in maxWidthVisibility().";
    @return null;
}